Shall it support all GPUs? Or just newer GPUs? What about the version of the Windows OS supported?
From Nvidia, DirectX12 supports all GPUs using the Ampere (RTX 3000 Series), Turing (RTX 2000 Series), Pascal (GTX 1000 Series), Maxwell (GTX 900 Series), Kepler (GTX 700 Series) and Fermi (GTX 500 Series) architectures, although 4000 series RTX cards should work as well.
It's important to note that where series are missed out (for example 600 and 800 series) they may be using architectures from older generations, for example the GTX 600 Series are actually using the Kepler architecture, and the 800 Series are actually only a series of laptop GPU, but should still support DirectX12.
On the AMD side, I can't seem to find a definitive answer, but just know that most GPUs in todays age will support it, and its not worth downgrading to DX11 for the tiny percentage of people that can't play it.
In terms of operating system again, I wouldn't worry, about it, DirectX12 works on Windows 11, 10, 8, and slightly on 7. But as I said before, anyone who plays games will likely already have a decent GPU and will probably be on Windows 10 or even 11 by now.
What changes when a new DirectX version comes?
Well, I'm not quite sure what you mean by this one, when a new version comes, it comes. If you're talking about what that will mean for your game, it would mean you're using an older version. But would you rather wait for them to release a new version? Probably not it would be a waste of time.
Hopefully that answers your question(s)