I think large tech companies want people to believe that old school forums are a thing of the past. This further increases their stranglehold on our attention and the information we have access to, which means more profits for them. In reality, for almost any topic, there are still old school forums alive and well.
To answer OP's question I looked up "mental health forum", and saw several that looked reasonable. You'd probably get more variety too if you try a few different search engines, e.g. Duck Duck Go, Google and Yandex. If you want one where you can openly discuss wanting to CTB without judgment, though, I doubt you'll find better than SaSu.
It's one of my goals to start using old school forums more because they don't wreak havoc on my attention span, they don't make me feel like shit, they feel more like a community, and they actually have higher quality information. (I actually learn things rather than just thinking I'm learning things.)
I just the other day I found a forum for transgender people at all stages (including pre-transition, transitioning, non-transition, de-transition) called susans.org, and so far it seems fantastic. I feel like being trans would have been so much easier if I'd found it sooner.