Just a day after the GTK+ 2.91.2 release, I managed to release gtkmm 2.91.2, with (hopefully) all the new API. I have Glom and the gtkmm-documentation examples building with the latest gtkmm as a test.
I also did a release of glibmm 2.27.1, matching the glib 2.27.1 release. JosÃ© Alburquerque has been working very hard on this, while also working on gstreamermm. But we really need some feedback on all the new API, particularly the Gio::DBus* stuff. Someone needs to sit down and write example code to see if it even works. We skipped the stable glibmm 2.26.x releases because this had not happened so we were not ready to call the API/ABI stable.