Honestly, Toxapex just feels limiting. There are so many Pokemon I want to try and experiment with using that just become invalidated due to Toxapex's exsistance. And yeah, every Pokemon has it's walls, but given Toxapex's inability to be worn down thanks to it's great recovery, being walled by Pex is pretty much a complete death sentence for any Pokemon that may want to try to squeeze its way into the OU metagame.
Yeah, I think it's fair to say Pex can have that effect on the meta. In the case of Pex, we actually we have an obvious example of how Pex can restrict teambuilding:
Keldeo. Looking at various data, one can see that Pex has almost single-handedly killed Keldeo's OU usage across 2 generations. A tier superstar in Gens 5 & 6, Keldeo experienced its first ever taste of UU in Gen 7, the same generation where Toxapex was first introduced. Sure, other new mons like Koko didn't give it an easy time either, but those mons didn't really want to switch into Keldeo, either; that was not the case for Toxapex (and Fini, but she was always a sleeper pick). Pex was easily one of the most prevalent defensive mons in USUM, to the point that it single-handedly made TSpikes a legitimate strategy in OU again; with that level of usage, it was easily the most common defensive answer to Keldeo.
Moving onto this gen, one would naturally expect Keldeo to have an easier time without many of the breakers that made USUM OU into mini-Ubers. And yet, despite being better in terms of viability compared to USUM (B vs. C rank), Keldeo still fell off the map and into UU
before DLC dropped. The Pre and Post viability rankings also show off this dynamic; as Pex has actually gotten naturally better following the DLC drop (A+->S), Keldeo has dropped off further (B+->B-). Sure, Kel also has to deal with Slowbro, Amoonguss and Grassy Glide Rillaboom now, but Kel dropped off before those entered the picture. Clearly, the root of Keldeo's inability to keep up was in the pre-DLC meta, a meta where Pex was still a great mon for most teams. So, what's different for Keldeo now vs USUM? Simple.
No Z moves. While they weren't an automatic solution to Pex, they did allow Keldeo to at least
theoretically do damage against the bunker. Keldeo's only answer to Pex now is Taunt- which is notably not on its best OU set, as Substitute is overall more useful for it. Even with Taunt, it still can't do much without a bunch of CM boosts, which can easily leave Kel open to one of Pex's teammates coming in. Also, it still doesn't like losing its item or getting burned.
Now, I get that Keldeo was kind of already on the way out of OU in USUM, and was kept proper OU in spite of itself. I also know that there were a few new additions that could beat Kel; however, every mon has checks and counters they have to deal with, so the 1 or 2 new threats should not have been a death sentence. The removal of numerous key threats thanks to Dexit should have opened things up for it, even a little, and yet that didn't happen. The usage did not reflect Kel getting better. The steady rise of Pex that we've seen thus far, a mon that can shrug off virtually anything the unicorn could do to it, and one that is currently bigger in the meta than any other solid Keldeo answer, naturally looks to be a key reason as to why this happened. Other than Zeraora (which only managed UU in USUM), it is the only mon that completely screws Keldeo in OU present in both generations (thus far)
and was not a factor before SM (after all, Keldeo's checks in BW & ORAS didn't keep it from OU stardom). Thus, it's not hard to see how Pex's domination has played a rather obvious role in Keldeo's decline.