BlueMooner: I'm probably going to have to do 10 soon, so I'm bookmarking this thread for later.
In 7, you could use "compatability mode" to help some older games work. Does this still exist / work in 10?
If you mean going to game (shortcut) properties and the Compatibility tab there, yes it is there also on Windows 10.
However, the issues you may face with some older games in Windows 10 may be different from Windows 7, My experience (since I install and play older, even CD/DVD retail, games on them both):
1. Windows 10 sometimes has performance issues or graphical glitches with some games that I don't see with the same game in Windows 7, for instance the GOG versions of Baldur's Gate classic, Icewind Dale, Might & Magic 9 etc. The first thing I usually try is to use dgVoodoo2 utility with these games, it seems to take care of quite many GPU-related problems with old games running in Windows 10. (With Icewind Dale 2 I actually used the ddwrapper utility to fix its graphical glitches and jerky scrolling in Windows 10, but I presume dgVoodoo2 could have worked as well.)
2. With some games there may be performance issues in Windows 10 fullscreen mode, like I had with a Steam game called Geometry Dash (it ran jerkily on Windows 10), even though it was a relatively new game. The fix in this case was to change the game display setting from within the game to either windowed mode, or "borderless windowed" or whatever it was called. Setting the game to true fullscreen in Windows 10 caused those problems at least back when I tried it.
3. The copy protection of many CD/DVD games doesn't work in Windows 10, they are blocked in Windows 10. By default they are blocked also in Windows 7, but in 7 you can "unblock" them manually. The same doesn't appear to be possible in Windows 10, so the only option in 10 to get those games to work is to find and apply some kind of noCD crack.
4. Microsoft seems to sometimes deactivate some older video decoders etc. which they don't consider relevant anymore, which is why e.g. Two Worlds' in-game videos stopped working in Windows 10 at some point. (Naturally the same would have happened in Windows 7 if MS had decided to do the same there, but apparently they didn't.). The only way to overcome that is to re-encode the Two Worlds video files to some other format that Win10 still supports, with an utility like ffmpeg.
5. There is one case where Windows 10 is actually better with older games: it doesn't have that "rainbow color" graphical glitch that Windows Vista/7 Aero caused with many games. However it is quite easy to get rid of that problem in Windows 7, there are several ways to achieve it.