Hi! Yes, Win7 support was dropped by Unity about 1.5 years ago. I don't remember at which exact version. Opening a project in a previous version of Unity is not officially supported. I mean, one can try that and go through the whole project and all the files, manually correcting internal meta files Unity generated and fixing any possible bugs that crop up. That's a lot of work. And it's something that one can never be sure about. One really would need to re-test the whole game fully, with much thoroughness, just in case.
Given the tiny and decreasing usage of Win7, it's just not worth the effort because I'm already busy working on new games right now and trying to spend time efficiently. Any time spent on this game is the time I will delay working on newer and more interesting games...
Why are you on Win7 btw? I think MS dropped support for it long ago as well. My last laptop with Win7 I sold at least 5 years ago. In fact, I'm not even using the old laptop with Win10 anymore.
Yeah, I know, this phasing out of support for older system is one of the downsides of using a major game engine. Unity also has commercial considerations about what platforms or older versions they have to support. Once something becomes too rare to care about (while the costs of support are still there), they just drop it and move on. On my side, I also can't stick with the older versions of the engine because new versions bring bug fixes and whatever. I just keep updating the engine regularly. And I have to put out updates for the game as people ask for new features or when I add DLCs. It's the dynamic process of digital publishing, especially with free casual games that have to have high longevity. Not like in the 1990s when one would print physical CDs that remain unchanged for eternity :)