I wonder if SourceTree will also drop Mercurial support at some point? That would be unfortunate.
I've been assuming so; the official page overwhelmingly describes it, explicitly, as a git tool, it's not like it has broad support (ex. for CVS, SVN, etc.), and honestly in their shoes I'd probably want to remove all that code that no longer is useful for working with any of their products.
As sibling comment notes, that's going away. Even when it was around (and in my mind is now replaced by sr.ht), the problem is that 90% of projects (not a real number, obviously) were using git hosted on github, so if I want to contribute, then I'm going to use git and almost have to have a github account. There were never a huge number of projects hosted on BB, and an even smaller slice of those used hg.