As far as I can tell there hasn't been any releases in 5+ years. Is it a dead project, or is there a fork of it somewhere with ongoing development?
It's a decent piece of software, but it's still missing at least one must have (to me) feature: the ability to rotate objects.
No, the Dia development is not halted.
The Dia editor has been overtaken by GNOME. Its hosted at https://gitlab.gnome.org/GNOME/dia and during first half of 2021 it had cca 20 commits from cca 10 different people.
Thus, Dia is still much alive.