If an arceus holds a poison plate, will it gain STAB from poison moves, and will it still get the boost from poison moves due to the plate?h
Yes because it becomes that type so using a same type move will still get stab.
The plate works the same as it gets a 20% boost on them moves just like any other Pokemon would.
Source: experience on po.
How to change your username
How to add a user avatar
Why was my post hidden?
Flags vs Downvotes
Official PokemonDb League
Type a Pokemon name, type, move or ability to see instant results.