What would be the last version to ZDOOM to support OSIRIS?
Currently you need a mostly up-to-date GZDoom to run it. It uses some features that are fairly recent. Theoretically there could be a version of Osiris that works on an older version of the engine, but depending on how far back you want to go, it could be a lot of work and could have features cut out. I don't really have any interest in doing that myself I'm afraid!