I use quite a few favorite of my favorite apps that were designed for XP on both Win 7 and Win 10. So far, the only program I cannot use was the old Photograv 2.1 version. I was never able to get it to work on Win 7 and have not tried on Win 10. (I didn't spend much time on it because I kept my old Win XP machine for any time I need to use Photograv.)
The biggest problem I have with still running my old Eudora 7.1.0.9 program, is it can't handle today's character coding such as "smart quotes" and other characters. For example, it will display an apostrophe as ’.
Those much-loved apps may still work on Win 10. All my other old apps work exactly as they did on Win XP.