They're quite rare as we don't have many viable ones, but Gengar and Grafaiai used to see play fairly commonly, though they've become a rarer sight recently. Toxtricity and Dragalge were never properly UU by usage, but they hit hard and can be valuable for setting TSpikes. There's also Muk that's a very niche pick, but being the only real specially defensive Poison type allows it to mess with Fairy+Magnezone cores since it can really get in the way of Florges and Sylveon without being trapped by Magnet Pull. But yeah even taking rare options into account, there are only a few viable poison types. There are some mons like Quagsire or Gastrodon that like using it as their Tera type though, so if you're looking for Poison's resistances, those can be useful. Low amount of Poison types also makes Toxic Spikes much more threatening and make it a prime option on Greninja for example. Might have rambled for a bit too long, but in short, Poison types are rare because they're not especially good, but you can still make them work pretty well if you want to.