Plenty of proprietary packages had robust communities well before anything like github existed.
In fact, github is arguably a poor substitute for a community. Look at how communities such as the ones around the linux kernel or cPython communicate. It sure as shit isn't through issues or pull requests. Or rather, perhaps, the limited technical communication that an issue tracker is good for is not a valid large community consensus building mechanism.
> The key definitional question is the license, not any of the cultural stuff.
For direct modification, certainly. For customization, maybe. For ignored issues like the parent to my original comment was bemoaning? Maybe LLMs are good enough that we are almost ready to say "Make me a custom version of Photoshop" and then it doesn't matter.
Yes. I've actually added some features to Blender like this, related to some highly specific use case. Just clone the source point an agent at it, say what you'd want (you have to be able to formulate this in a halfway coherent way), and it succeeded. Now, I haven't thought much about what it would mean to maintain this, but I think if I need this ever again, I can point an AI to it again and say "do the equivalent change now to the latest version". For occasional use this is fine.