Let me start off my perspective on Heat Rock, and Fire as a whole, with a personal anecdote on my pet cat.
See, one of my cat's is named Bug, and while Bug is a perfect loaf of a creature, for the longest time he was also quite the fucking idiot. So much so, that for a good while our nickname for Bug was "Lobotomy Cat", this was due to the absent look in his eyes that suggested a complete lack of brain functionality and general dumbassery. We'd see Bug stick his face in a bowl of walnuts and raise his head in confusion with cheeks full before falling off the table. The depth of Bug's persona is beyond just being aloof and absent minded though, as a taste for fresh air has developed in Bug a tendency to try and run out the door. This quality of constantly trying to run away has given him a second nickname: "Deadbeat Dad". Now just as Bug seems pretty aptly described as a braindead creature that constantly tried to run away, so too is Fire a fully braindead type where morons can run away with the game fully on the merits of mindlessly clicking.
Now, while non-mono mains who don't know better try to portray the tier as nothing more than Russian Roulette, Fire is perhaps the one case where they might be correct. You load fire, you mindlessly click for 15 turns, the game ends and you're dumber for it. Watching TV all day turns your brain to cheese, and so does playing fire. Obviously this in of itself does not warrant regulation of the type, it's the most mindless of types, but that in itself isn't a problem. The issue with fire comes from the addition of FirePon, Ceruledge, and Volcarona in 8 turns of Sun. It's not just a matter of "mindlessly clicking", it's mindlessly clicking with 3 of the most threatening setup sweepers in the tier supported by Healing Wish and 8 turns of Boosted Fire Moves + Half Damage to Water Attacks. Some types can handle this without much issue, others I feel get stretched thin in this metagame. I see these 2 things in combination as part of why Heat Rock is being suspected. Covering fire in the builder can feel like a contradiction, and part of the problem with fire is fully just that. The effort to cover fire, for many types, can require undermining multiple other matchups for little other benefit. Covering fire to me, as a result, often feels more like a choice than an optimization.
Now I just finished getting reqs, and shamefully, getting reqs took me 2 attempts. My second attempt was pretty easy, I just spammed ground and got reqs in 24 games. My first attempt, however, started off with me spamming Poison, and was followed with a switch to Dark and Dragon where multiple back to back 10% misses caused me to bin the run. Now as many people know I kinda just use whatever I want when I play or ladder, this includes going through every type as well as engaging in shenanigans like Specs Ttar Dark, but also I'd say has led to me getting a decent enough look into every type and into building for every type in Gen 9. As Arken said, and I agree, the extra turns of sun exacerbates difficult matchups. In addition, I think it's a pretty well shared experience where the late game vs. fire is lost because of the extra 1-2 turns that sun brings. It's not like you can't point to how Heat Rock has won many games for fire.
As I stated earlier, covering fire can often feel more like a choice than an optimization, and frankly speaking I think this is the best way to put the situation. I'm not even talking about nonsense like Ability Shield Tran, anyone who even tries bringing that up is either being dishonest or doesn't know what "overcentralization" means in the tiering philosphy. Rather, real things that exist to cover the fire matchup are for example running pokemon with no item on Poison. Itemless Pecharunt, and even consumable item (sitrus/red card/air balloon) Toxapex as a way to counter Ceruledge, are techs brought up when people ask how to improve the fire mu. When you ask why Tyranitar is listed in the VR for Dark, the answer will be "It has Thunder Wave, it has Knock Off, and it's better into Fire than Ting-Lu". Likewise, a big boon of Roaring Moon is how it helps with fire. Frankly I think Roaring Moon is pretty bad on dark otherwise as far as matchup spread goes. The Poison Team I used when I first started doing reqs was a more recent one I made for a solid Fire MU and to better answer Raging Bolt, and the consequence was Flying + Ground both going from playable to loss on preview as well as a more shaky steel. When you see Cee bring Water on the ladder the team is 5 stall mons + Gren, when you see the same type of Water brought in major tours Walking Wake replaces Gren half the time as the team's prep deemed it essential to not lose to Fire. I say all this because I don't think there is really a lack of options to cover Fire, rather I think Fire forces builders to choose between an optimized team that can handle top types like Flying/Dragon/Steel reliably but is shaky on Fire, or a team which is less reliable into those 3 types but wins vs. Fire. Naturally, people decide to cover Flying/Dragon/Steel over Fire, just as people decide to run Ting Lu instead of Tyranitar.
Currently, I think I Lean Ban though I am a bit on the edge. I don't see Heat Rock as healthy to the game in the longrun way things are anyhow. I'll also say that having as much mon diversity as possible is also what I see as one of the core tenants of monotype, considering how big of a limitation Monotype is to begin with. Banning Heat Rock seems not only a decent way to make Fire Healthier, but hopefully prevents people from wanting to get rid of HotPon and Volc as well which have both been brought up many times. Either way, if Heat Rock isn't banned now I fully expect people to turn around and cry about Ogerpon/Volc/Heat Rock once MWP starts and we're reminded of Fire's impact on the tour scene.
