Windows 7 does not receive security updates anymore, so its use should definitely be discouraged even if it "works".
For software devs, they almost certainly don't want to support an obsolete OS with a small number of users, as that requires time and effort on their side (e.g. if a user has problems on Windows 7 what should they do?). And if they want to refactor some code, do they really want to test on ancient OSs and add needed workarounds / compatibility fixes?
It's not though. It's wildly against it. The spirit of open source is that anyone can take open source code and use and modify it. This isn't the case here.