For those of you who recently got into UUbers, I would like to to give a quick run-through what Giratina did and why it was considered vital to the tier function before.
i: The Tier King 
Giratina until October was arguably the single best pokemon in UUbers, and a near mandatory pick on practically any team that was not Hyper-Offense. Its abiltity to act as simultaneously the best phaser, status debuffer, and tank in the tier while being the best Ghost and Fairy type (through tera) let it have a unique ability to "soft-check" most of the tier, and neglecting to bring it would require one to bring multiple sub-par pokemon to make up for the sheer utility Gira offers, and these teams were practically never seen for a very good reason.
It also had the ability so effectively soft check almost every single common offensive threat in some way due to its huge stat pool and access to both T-Wave and Will-o-Wisp that can cripple most set-up sweepers already, and then phase them out with Dragon Tail or Roar to neuter their boosts. Notably this allowed Giratina to hugely limit the booster energy sweepers in Gouging Fire and Roaring Moon, which often relied on that boost to even have a chance to break through Giratina itself. This meant it was regarded as one of the main things holding Hyper-Offense in check, using its disruptive tools and Rest to be a long-term answer to this brutal playstyle; which notably was the playstyle that lead to the last ban before Giratina left in Spectrier. Sticky Web would also receive one of the highest survey scores in months in the following survey, showing that despite Giratina's prescense Hyper-Offense was clearly still problematic.
Giratina was also a defining aspect of how bulkier playstyles were able to function, as its sheer bulk and ability to ignore status and even stall through pressure lead Giratina to be able to provide consistent counterplay to many defensive threats. This was huge in enabling the success of Bulky-Offense styles, who greatly appreciated Tina's ability to put huge amounts of pressure on many defensive arceus forms, and the experimentation with Alomomola, Pecharunt, and Lugia (to name a few). Even Garganacl was not a consistent answer due to a well played Tina being able to drain it of all its PP through pressure. This isn't to say that balance was unviable, but its viability was clearly stunted when it had to compete with its own best pokemon - let alone the demonic offensive threats that lead it to be mandatory.
ii: Overcentralization - Is it an Issue? 
Through this depiction of Giratina, I think regardless of your stance on it one thing is clear; Giratina was easily the most overcentralizing Pokemon in the tier. Every teamstyle was dependent on how it could beat Giratina, while somehow managing to survive the offensive onslaught that the tier consisted of. Leading up to October, many of the meta developments made all centered around Giratina and it's role in the tier. Including (but not limited to)
- Scizor

- Scizor was able to be a powerful priority user that preyed on not only tera fairy users aiming to avoid Giratina's Dragon Tail, but to threaten Gira with Knock or Bullet Punch after tera fairy.
- Dragonite

- Dragonite emerged as an incredibly potent anti-offense tool for offense teams of all flavours, namely due to how it could use its versatile toolkit to circumvent opposing threats with E-Speed and check Giratina with super-effective STAB, Lum berry, Encore, Tera fairy, the list goes on.
- Arceus-Ice

- While Arceus-Ice has been known to be strong for some time, after the departure of Zacian Arceus-Ice established itself as one of the best Arceus forms in the tier due to its ability to be one of the best anti-Giratina Pokemon in the tier, while being able to utilize its STAB and tera versatility to threaten almost the entire tier.
As mentioned, Giratina also hugely impacted the development of balance and bulkier playstyles. While after its ban we saw a meteoric rise of to-be staples such as Pecharunt, Alomomola, or even niche options such as Sinistcha and Clefable, Giratina itself was able to counter many of the potential defensive tools in the tier, which ultimately began to put a strain on balance builders to handle it. Some success was able to be found, with some teams being able to capitalize off of tools like Mandibuzz or Corviknight due to their ability to pivot and posessing decent tools into the rest of the meta. This creates a strange scenario where Giratina was the main thing driving bulkier styles and offense's need to account for it, while simultaneously being the most limiting thing to the overall success of these styles.
So when does this become unhealthy? In my opinion, overcentralization is something that should come to be expected of tiers with this power level, as we've seen consistently in both Ubers and UUbers. But the line should be drawn where something turns from overcentralizing to metagame-warping (in an unhealthy way). I personally see something to meet this criteria when it changes from being a primary threat in the tier that most teams will have to account for in some way (eg, Zamazenta in OU) to whenit forces teams to be largely centered around how to check, and ultimately remove this threat. This was never impossible with Giratina, as it's omniprescense lead many to find ways to effectively pressure it long term. But this can largely be observed through its impact on the teambuilder, where not being effective into Giratina was often a reason to simply not include something, with the upsides having to be huge to even consider.
This ultimately lead me to believe that Giratina met a threshold for being an unhealthily-warping presence in the tier, as the way it warped the metagame was something I deemed harmful in multiple ways.
iii: The Crimes of Giratina
1. Hazard Control
Giratina does have defog itself, but the issue here is that it almost solely dictated hazard control in the tier. Its existence made rapid spin almost impossible to use (especially after the departure of Terapagos), and with defog being incredibly limited in distribution you weren't seeing it be clicked particularily often. This meant that by virtue of existing, Giratina was able to create an environment where hazards were able to thrive, due to it being the only thing effectively able to remove them. Despite this, it often was not able to because of it's moveslots being in incredibly high demand in an attempt to limit itself from being too passive. This created the perfect storm for hazard stacking to be an incredibly effective playstyle, not just in bulkier teamstyles but in...
2. The Polarization Towards Hyper-Offense
Hyper-Offense hugely benefitted from the lack of hazard control options, leading to spikes offense and sticky webs being contentious points of discussion, especially after the Zacian ban. The difficulty to manage the hazards that gave these teams such an advantage allowed them to flourish, but created a rather noteable issue at the same time, which was the H/O mirror. While these are nothing new, the ways in which teams would adapt in an attempt to manage Giratina through unconventional tera types and moves would often run the risk of leaving them open to opposing hyper offense, leaving many players with a hard decision - answer Giratina effectively, or manage the mirror into H/O? For a while, this was accomplished through Terapagos as a secondary lead/phaser when it was around, but after its departure the balls-to-the-wall offense strategy ended up being the best. Priority became more common then ever before, and investing into the demonic special threats of Arc-Ice and Electric became the norm. Games slowly became decided more and more on matchup, who haad the better teras, who had the movepool alterations that happened to be best, etc. H/O became incredibly inconsistent, but at the same time was significantly more consistent to the majority of the playerbase compared to Balance, which was a brutal and punishing playstyle to adapt.
3. Pushing Defensive Counterplay Away
As I have said a few times now, one of the most notable impacts of Giratina was how it was incredibly punishing of many other defensive mons, because they would rarely have the damage or longevity to beat it. Rest sets would be impervious to toxic stall, and anything that could potentially handle it longer term like Magearna or Psychic Noise Lugias would have to carefully manage their PP, where one slip up could be game-losing. This created an environment where many players consistently complained about the difficulty in running Balance and Bulky Offense structures, because in short it WAS difficult. Not impossible at all, but Giratina put such a ridiculous amount of strain on bulky play that few outside of the most commited players were able to get consistent success with the strategy. You could argue that this promotes skillful play and building (which I would agree to), but the prevalence of cheesy and unpredictable hyper-offense didn't just effect mirrors, with how much Bulk had to manage Giratina ON TOP of all the offensive threats, it was not uncommon for some matchups using similarily well-built teams to be unwinnable on preview for the bulk player.
iv: What Gira-Less Taught Us 
To preface, I do not and never have believed in theory and looking at reasons beyond the current meta to make decisions. What I realized over the course of the past 3 months is simply identifiying the issues I had with Giratina when it was present, which I could more easily put into words after seeing what a meta without it looks like. I agree with Slikkles, I happily viewed Giratina as a necessary evil in the tier due to how it functioned as a "glue" despite its flaws, believing it to be the easiest solution to handling the instability that has consistently arisen in UUbers. However, when it was gone (and arc elec likewise) the first thing that I noticed was the stability of the meta, and how so many things that felt like huge flaws from the tina meta seemed to solve themselves. The most shocking development was how much speed control in the tier improved, with Deoxys reaching new heights of viability, and Scarf Great Tusk + Zamazenta. Of course priority got better (and became stronger), but this was largely to be expected.
But I want to continue with that discussion around stability, because in the time I have been playing the tier I don't think there has ever been a time where I haven't felt pressed to encourage for action on something. While I still have my issues with how strong Roaring Moon is for example, I feel that removing Giratina and its centralizing effects left the metagame in a place where finally, if something felt too strong the metagame could adapt to it. Giratina was so overwhelming in shutting down other options that adapting to metagame threats became less focused on managing the threat it self, but determining if an innovation could withstand Giratina at the same time.
It especially illuminated just how many things Gira invalidated to me (I know I keep harping on this point, but I feel its important). The sheer diversity of mons we have seen had me reflect on Gira meta and the pieces that weren't able to be used then, and why. All of it (shockingly) lead back to Giratina, where it recontextualized Giratina not just as the clear staple defensive piece of the tier, but definatively the most warping piece of said tier after Zacian left. The impact it had was not felt in how it neutered offense as the tiers best wall, but in how it made the rest of the tier grapple with ways to manage Giratina itself.
Conclusion: Banning a Defensive Staple 
This isn't something that happens often. When it comes to offensive threats, it is easy to pick out mons that are too strong, hit too hard, and have no consistent defensive counterplay. We've seen this play out time and time again, but Giratina is clearly not the same. My gut when I think of a "bannable" defensive mon is to think of something that is quite literally unbreakable by the rest of the tier, but Giratina doesn't meet that either. Instead, Giratina's issues as it pertained to the tier consistent of many smaller issues, most notably how it effected the tier around it, not how Giratina itself was over-performing (though perform it did)!
I felt ultimately with how much Giratina warped the tier and indirectly created instability by existing that the direction to take to push for a healthier, more stable, and more competative meta was to ban it. While issues that aren't Giratina do exist, it has become increasingly clear to me that Giratina is the ultimate culprit for what allowed an unstable, unbalanced, and ultimately unenjoyable metagame.