I agree that Porygon is potentially problematic in the current metagame, but I don't think suspecting it addresses the root cause of the issue.
I'm going to be drawing comparisons to Agility/Choice Scarf Porygon in XY because this was the last metagame where Porygon was allowed. I'll only be referring to some broader / more obvious trends, so hopefully this isn't an issue.
---
The first thing to establish is that in XY, Porygon's dedicated offensive sets - Agility and Choice Scarf - had been explored and established, but repeatedly failed to gain serious popularity throughout the generation. From a set vr perspective, Agility Porygon was seen as a C+ or B- set, and Scarf Porygon was more along the lines of C- or C. Let alone suspect-worthy or top tier, these sets were not even considered good - they were barely on the cusp of viability.
Now, if we were to create a modern SS set vr, we would probably see something like Agility Porygon in A or A+, and Scarf Porygon in A+ or S-. They both rank among the most threatening sets in the metagame; Scarf Porygon in particular has risen from Mankey tier to Mienfoo tier. (As always, a vr isn't an argument for or against a ban on its own; I'm just using it to illustrate the sheer degree to which the offensive Porygon sets have improved.)
However, all the tools to deal with Porygon that existed in XY still exist now. In fact, Porygon itself actually lost a helpful move in Hidden Power while mechanically gaining nothing. So what changed? A lot of things, of course. These things can overall be summarized as a shift to a highly physical-centric metagame - that is, the metagame strongly leans toward investing in Attack / Defense, and generally neglects Special Attack / Special Defense.
The most immediate way this shift has helped offensive Porygon is that it's naturally stronger - Porygon struggles much more to OHKO and 2HKO threats in a metagame where mons can afford to invest in SpD. Back in XY, sets like 14 SpD Timburr and Mienfoo were standard, and defensive sets like (sp)defensive Spritzee, Eviolite Vullaby, and Trace Porygon were common too. A set like Scarf Porygon could only 2HKO or 3HKO these sets. Meanwhile, Porygon's lack of resistances meant that without Eviolite, it would have very few switchin opportunities. Between a lack of sweeping potential due to low damage and very limited switchin opportunities, offensive Porygon sets just weren't usually worth it.
Porygon also gains a second benefit from the physical metagame, which is that Download is much more reliable. Back in XY, most teams could easily afford at least 2 mons that could deny the SpA boost, which meant that denying Download was a frequent counterplay opportunity against Porygon. On the other hand, in the current gen, it's not uncommon for all 6 mons on a team to provide the SpA Download boost. Without Download, Porygon hardly has the power to sweep - its Tri Attack is as strong as Pawniard's Iron Head.
The unreliability of Download was why bulkier Porygon variants were so much more popular in XY, as they could provide consistent defensive value instead. The defensive sets - bulky Trace and bulky Download - were what Porygon was S-ranked by in XY. As a defensive mon, these sets don't really benefit from the shift to a physical metagame. In fact, the generally fast-paced metagame means that they have outright gotten worse.
This is why Porygon itself has remained around equally viable from XY to SS, even though its offensive sets have leapt from being barely viable to potentially suspect-worthy. I would like to establish now that the remainder of the post solely discusses offensive Porygon - defensive Porygon simply doesn't provide a relevant enough boost to Porygon's viability as a whole to be worth mentioning in a suspect discussion.
---
Let's break down the shift to a more physical metagame from XY to SS a bit further.
We can attribute this overall physical shift to three major trends, each of which greatly pushes the metagame to favour the physical spectrum. If you take away any of these three trends, and make investing into special bulk viable again, then offensive Porygon would suffer accordingly. And remember, there is a LOT of room for offensive Porygon to suffer, seeing how its offensive sets went from hovering C/C+ in XY to hovering A+/S in SS. Porygon certainly isn't Misdreavus or Gligar-tier broken; I'm confident that taking away even one of these trends would reduce Porygon to something that is completely reasonable.
1. Mechanical differences.
This covers the actual mechanical changes to the LC tier from XY to SS. The most notable of these is, of course, the loss of Hidden Power. This made the entire special spectrum weaker; mons like Abra and Magnemite were badly nerfed by the loss of valuable coverage moves, and the metagame adjusted accordingly to their lowered threat level. The important thing to note here is that some special mons were more badly hurt than others. Mons like offensive Porygon, Vullaby, and Gastly did use Hidden Power, but weren't actually very reliant on it for coverage; so when the metagame shifted away from special attackers as a whole, these mons actually improved overall, in a metagame that was now less prepared for them, to become even more threatening than before.
When I discussed this with dcae, he brought up another indirect mechanical difference: the loss of both Hidden Power and Pursuit, coupled with the reduced Knock Off availability at the time, was enough to push Gastly to a ban. While Agility Porygon doesn't care for this one way or another, Scarf Porygon greatly appreciates being able to spam Tri Attack more consistently. This helps to explain why Scarf Porygon is arguably the more threatening set now despite Agility Porygon being seen as a bit more justifiable in XY.
This is probably the most important piece of why offensive Porygon has improved so much. If we simply shifted the metagame back to its XY appearance, then the mechanical differences alone would likely be enough to prop Agility and Choice Scarf Porygon up to B+ or even A-. Of course, we can't simply revert mechanical differences, so offensive Porygon being at least a B+/A- threat is just something that we should accept in the SS metagame.
2. Vullaby.
The second trend is also very important, which is the introduction of an extremely prominent physical (or primarily physical, in the case of mixed sets) wall-breaker that makes its way onto about 70% of teams: Weak Armor Vullaby. The bulky Fighting-type archetype has historically been the most prominent group of the physical spectrum since BW, and physical Vullaby alone provides a nearly comparable presence. This ensures that investing in physical bulk is almost always more important than investing in special bulk when both options are available.
Vullaby doesn't just push the metagame to be physically leaning, however. Vullaby as a whole - physical, mixed, and special - simultaneously ranks among both the most effective wall-breakers and most threatening sweepers that LC has ever seen. This makes defensive archetypes much more difficult to build and justify, meaning Vullaby single-handedly pushes the metagame in a noticeably more offensive direction. This is something else that offensive Porygon greatly enjoys.
3. Grookey.
To start with, I do not think Grookey is broken right now. Nearly every typical team includes a Vullaby and a bulky Poison-type by default. If the team has just one or two more mons that can live a Grookey OHKO, then we're consistently reaching nearly XY Fletchling levels of preparedness, even though on paper Grookey should be much more difficult to prep for.
It's in this sort of metagame that Porygon thrives. It can badly threaten Grookey checks with its coverage moves, and loves that the generic EV spread is more likely to lean toward physical bulk to improve Grookey rolls. Like most other top tier mons, it's used around a third as much as Vullaby, but with its access to strong priority, it probably forces physical trends more along the lines of half of Vullaby or the bulky Fighting archetype.
On the other hand, if we suddenly reverted to XY trends, with Fairy-types as the primary Fighting-type answer and everything running 14 SpD, then offensive Porygon would fall back into near-total obscurity. However, Grookey would basically be doing what it did when Life Orb first gained popularity except even stronger - it would be pretty ridiculous.
In other words, I see Grookey and Porygon as a bit of a seesaw, where as long as both mons are in the metagame, then if the metagame is sufficiently prepared for one of the two, then the other is sure to be quite strong.
With that being said, if we see Grookey and Porygon as equally weighted, comparable threats, then it makes more sense to focus on the mon that's currently the issue, which is Porygon.
---
So that leaves Vullaby.
The Vullaby suspect happened a few iterations of the metagame ago, and at the time, I argued that Vullaby was banworthy on its own merits. However, the rise of Porygon has brought to light another angle with which to approach Vullaby from: it is also incredibly efficient at overloading its teammate's checks.
Remember, this isn't a mon that just chip its answers, it flat out 1v1s them and wins. A typical typespam overloading core, such as fightspam, simply looks to weaken shared checks until they can't switch in anymore. Some glass cannon overloading cores, like gastbra, are able to outright KO their checks on a correctly predicted switch. Vullaby goes well beyond this; it 1v1s its most common answer, Pawniard, and comfortably wins. This is of course fantastic for Vullaby's own sweeping potential, but it is also equally effective at setting up a sweep with a teammate like Porygon.
This isn't just limited to Pawniard. To begin with, Porygon's switch-ins are as few as its switch-in opportunities - it doesn't have much of either - but once Vullaby is through, its switch-ins are reduced to zero, and even its revenge-killers become limited in scope. Pawniard, Ferroseed, defensive mons, you name it; if it exists, Vullaby alone is sufficient to bring it into Porygon range.
So not only has Vullaby's presence established significant trends that severely push the metagame to be both more physically leaning and less defensive, to the benefit of offensive Porygon, but it is also among offensive Porygon's most valuable immediate teammates.
In short, all roads lead to the mon with 85% usage.
