Glib 2.51/52 is meant to just target glib 2.51/52, with no API or ABI
break. glibmm-2.54 is the ABI-breaking API, which will be used with
gtkmm-4.0. It's not impossible that this will change again, I'm afraid.
I'm not aware of any API/ABI breaks in glib.
> Maybe then, based on this decision, that stuff should just be put in
> glibmm-2-50 anyway?
I don't think there is any issue about where to put glib/glibmm API.
It's only unusual for GTK+/gtkmm.