It has git, svn, Perforce and Bazaar in the list of plugins, but not Mercurial. Maybe there's a way to get it but I asked in https://forums.gentoo.org/viewtopic-t-1101622-highlight-.html?sid=31cb80ea3844d9ea57f030f6b3d831ce and received no answer.
I have mercurial as a global use flag.
kdevelop-mercurial package was dropped from kde overlay in 2015 for failing to be ported to KF5. It never got a release upstream and as such was never provided in Gentoo ebuild repository.
Hrmph. Better close this 'bug' then.