There's not much to do about it other than move on right? It's their choice. I guess though, it would be nice to have received a message rather than have them simply disappear but it is what it is, and life will go on.
My other brother posted regularly but when life needed his attention had simply stopped. The account is still there and he (or someone on his behalf) may do something with it, but it'll be an anonymous account by his choice. (Maybe you already interact with that exact account.)
Lol an air of mystery! Perhaps I do……. 🤔