By "designed for Windows 7" do you mean:
Not a UWP App
Only support D3D9, or include D3D9 along with D3D10/D3D11 although some games for 7 were D3D10 or 11, the vast majority were not.
Developed with a compiler that did not support Windows 10 at the time the game was developed.
Only includes Windows 7 and below APIs.
Primarily 32bit but may have a 64bit executable but some might be 64bit only.
Game released before Windows 10 was released but games released after would likely still apply since they are in development for a long time and likely were not developed on Windows 10 machines.
I heard a rumour that GTA V stopped working on Vista due to CEF (not the steam client) but I haven't verified it yet . A very old game getting updates and dropping OS support is very rare since old games are rarely updated but GTA V makes so much money so it's one of the exceptions so I doubt it's worth worrying about overall. Of couse the online store client dropping support is a different matter.