git and some kind of simple collaboration (like the "Teamwork" extension, that is, unfortunately, no longer working)

Archi the archimate modelling tool has a great way to break complex XML exchange files into small files which can be stored in git with a merge tool that understands the file format parts. Basically each entity within the exchange file format is pulled out into a seperate file and stored in git. The the actual exchange file is rehydrated and imported to the tool. When the time comes to save, a commit is made, this rips the exchange file back into discrete files, finds the ones that changed and saves them, then git commits them.

Sems StarULM.Teamwork is no longer maintained and current does not seem to work...is anyone activity maintaining it?

What about this extension? https://github.com/DaftPoint/StarUML.Teamwork

Maybe best as optional plugin. Others might prefer mercurial or even no versioning within the modeller but have a slick interface (especially beginners).