I've heard of Proton a few times but last time I checked only 600 out of Steam's 50,000+ games are compatible with it. Not bad by any means but using an older Windows version still seems to offer more at the moment.
I have 400+ titles in my Steam library and play most of them on a Linux machine. Nowadays it's very rare to find one that does not work out of the box, and in those rare cases, there's always glorious eggroll to try.
Check by yourself on protondb.
Using an older Windows version to run Steam is a really bad idea. Steam requires an internet connection, old versions of Windows, are by definition, not up to date.
