We have not released glibmm 2.52.0 yet (a version of glibmm-2.24). It's
still unstable - the latest release is 2.51.6. Theoretically it should
have been released already, but we didn't do that due to the GTK+4
confusion. We should release 2.52.0 fairly soon, I guess.
https://git.gnome.org/browse/glibmm/log/?h=glibmm-2-52
Now would be the right time to add or deprecate API in glibmm-2.24,
before we make that stable release. However, we cannot break ABI.