The general directories I have found are mostly just compilations of forums rather than curated lists listing just the best ones. You best bet to search for them would be looking for "awesome"-type resource compilations for the particular niche you are interested in, and then seeing if that list has an entry for "Forums" under its resources.

I haven't seen anything like this before, but I think it's a good idea. I would suggest starting it yourself and putting it on GitHub something similar so that anyone can submit more via pull requests.