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.
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There have been no updates for years, and the developers haven't responded to inquiries, so the project does appear dead.
I haven't found any forks or any true open source alternatives. Inkscape can be used to accomplish anything that Dia can or should do, but it's a bit like using c++ to write and format a word document.
Edit: draw.io seems to be a decent alternative
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.