As someone who is basically done with transition and stealth, I can tell you exactly why they disappear.
It's called living life.
When you're in transition, being trans is kind of all that you can think about. Once you're finished with transition, wherever you might deem that finish line to be, an amazing thing happens. You can start living your life without gender dysphoria getting in the way. People you meet for the first time can't even tell that you're trans anymore, your body is more or less completely that of your identity gender, and frankly it seems stupid to even think about being trans anymore, because you stop seeing yourself as trans, you just start seeing yourself as a woman/man first, trans maybe 4th or 5th on a list of things that you are. Honestly still being in transgender support networks becomes a scary thought, because what if someone sees you, and you're outed? That's usually what's happening with people who completely delete their videos from Youtube, or delete all of their pictures from a trans support site, is that they're becoming afraid of being outed, and they don't want to be seen as trans anymore, they just want to be seen as an ordinary member of their identity gender.
I've been here for almost 2 years now, and watched a lot of members come and go. And in almost every single case, the members that go are leaving because they're now post-transition, full-time, and thus they're ready to leave the support community and just get on with their lives.
Just about every single transgender community has this cycle... people are there for about 1-2 years or so, and then once they're passable and full-time, they all leave and a brand new batch of trans-people comes in to take their place, all with the exact same questions and the exact same fears and concerns that they had in the first place. Rinse and repeat. Only a few stay behind, usually specifically because they want to keep helping others out.
And again, it's not because anyone is having regrets. It's the opposite. It's because for the first time in their lives, they don't feel weird anymore, don't feel enough bodily dysphoria to need support anymore, and thus don't feel the need to even talk about it anymore. They're just out living their lives.