There was a discussion about the www.gtkmm.org
website in January. For instance
https://mail.gnome.org/archives/gtkmm-list/2022-January/msg00007.html
There is now a Gnome issue at https://gitlab.gnome.org/Infrastructure/Infrastructure/-/issues/848
about the gtkmm.org domain. Perhaps it will be possible to
transfer ownership of the gtkmm.org domain to the Gnome
Infrastructure team and have it automatically updated when the
source code at https://gitlab.gnome.org/GNOME/gnomemm-website
is updated.