Steam aren't just "no longer supporting win7", they're actively disabling all clients on that OS end of this year. As in pushing out an update that will fuck it up. Your game you paid for can't run without the client? Tough shit. No, you don't get to run offline or standalone. You will use win10 or go and get fucked.

Their reasoning is something something google something which is even fucking worse afaic, why the hell do google need to be involved in me playing a game that I've bought?

GOG actually offer both their gog galaxy (like steam client) or standalones for anything you've bought, with the advisement the latter doesn't give you online community features.