I believe that Pokémon themselves are never playtested at all considering the vicious cycle of intentional power creep. If playtesting on the Pokémon happened after say Gen 2 or 3, the power creep wouldn’t be so bad so quickly.
To be fair at this point one big question would be, how exactly do you "playtest" a pokemon?
Like sure, you could try to stage some PvP matches between the devs or some testers, but that's the same problem that big MOBAs/MMOs have where you only get a limited samplie size who also has confirmation bias to deal with.
One option is to do what for example League did occasionally, to invite over pros to actually check their designs for some champions and gather their feedback, but even then, *eventually* people find out ways to break stuff that would look balanced. Remember when everyone had a laugh at how terrible Spectrier was going to be and then it released and it got itself nuked from OU for being able to single handedly terrorize the tier with a single attack type? And how the reverse happened for Regieleki, when everyone was like "zomg this thing is fast and strong must be broken" and then now it barely holds OU usage and it's only a support pick in VGC.
And playtesting "for ingame" is almost pointless as due to the nature of pokemon games, pretty much any Pokemon can be used no matter if you're a complete newcomer or a pro, due to the games inherently being very easy.
Right now it’s like GF can’t really decide whether or not they should really care about balance - not making all Pokémon equally viable but keeping the weakest from being complete garbage and the strongest from being disgustingly annoying to deal with - or if they, in the end, are all about style over substance.
Honestly I think until before SwSh they werent particularly bothered by "game balance". Yes there's been a few cases of generational nerfs but they seem to not mind the power creep at all, in fact support it by releasing progressively stronger legendaryes every new game.
They are more interested in creating designs that sell and are well appreciated by the intended playerbase (aka kids and young adults), pleasing the competitive fanbase is just an extra. They have thrown a few bones to competitive players with SwSh, mainly with QoL stuff and ways to help
Which also really isnt exactly a huge deal, to be fair, you know what I think, that even if they were to limit powercreep, due to the nature of competitive, it'll always have a handful of particularly strong pokemon with > 70% usage, and then everything else being niche or worse.
You can change who the pokemon with 70% usage are, but won't change the results.
(I don't even need to search for a proof, you can look at any given Smogon metagame, and you'll have a handful meta defining pokemon, 10-20 "strong but not quite S tier" pokes, and rest being fringe or niche or unviable).
It's just the nature of competitive, if you're playing to win, you'll usually use the very best and not purposely nerf yourself. Even the famous Pachirisu usage wasn't out of "hey it's cute I'll use it" but was a calculated pick because Pachirisu just happened to have all the qualities to dick on the competitive meta of that year.
Overally, I think Pokemon is in that weird spot where unless they actually start to actually do balance patches, it's kinda pointless to bother with balancing it, cause realistically it doesnt matter at all, and due to it not having microtransactions that would want you to "keep people playing", all that really ends up mattering is initial sales and eventual future sales (which are usually supported by cross-game compatibility, pokemon home transfers, and game save rewards).
Incidentally, lack of balance patches is also one of the aspects that's severely gating Pokemon from becoming a real E-sport, but honestly, I really doubt GameFreaks or TPCI would be able to handle a real pro-scene for it anyway.