i remember seeing this suggested awhile ago, but it got turned down, from what i remember.
Yeah, there was a discussion a few months back and from what I recall, it was rejected because there's not enough threads to warrant it's own board - and I agree. I'd love a board just for Pokemon trades, Pokemon are the only thing I spend TBT Bells on, I love buying event Pokemon and shiny Pokemon - so having it's own board would be great for me and make things so much easier - but there aren't nearly enough threads for one. Saying they clog up the board is a lazy reason, I've never had an issue finding any threads for Pokemon (remember, you can click on the prefix tags to sort threads into Buying/Selling/Shop etc..), and on the flip side, I don't have an issue finding collectible threads either (not that I buy them). Again, with sorting through the prefix tags, and then the fact that there are ample collectible buying/selling threads, from a glance it doesn't look that hard to find a thread for a specific collectibles you're looking for. For both, you can also use your browser search function (Ctrl + F) to make things much easier.
There's an abundance on the first two-three pages, but once you get past those, they die down considerably. You also have to take into account that the reason for the influx is because of the 20th anniversary codes - it's easier to buy a code on here than it is to go to the store and get one there. If you exclude the threads that are after the mythical Pokemon (which will be gone by the end of the year), there's barely any Pokemon trade threads. I just had a quick look, on the first three pages, there are 3, 6 and 7 threads respectively, and then one each on the 4th and 5th pages (and that's including the mythical Pokemon threads, exclude those and there'll be less).
There's also the Pokemon Trade thread in the Pokemon board which barely gets used (probably because it's easier to post in the TBT Marketplace because they get more attention). If it ever became such an issue that there were too many Pokemon threads, an easy fix would just be to move that thread to the TBT Marketplace for people to post in when they're just looking for one or two Pokemon, rather than making a whole thread. Then if there was still a large amount of threads after that, then I'd say there'd reason for it's sub in the Pokemon board, but like I said before, right now, literally no reason (other than convenience), they're nowhere near as spammy and clogging as people make them out to be.
Though with Sun and Moon releasing in November, there could be a huge boost of threads, and maybe open up more opportunity for one, but right now, I think it's fine.