Is it abuse? Are there rules about what sorts of media can and cannot be protected by DRM?

Yeah, technically this seems like exactly a DRM scenario: Party A sends information to party B intended for use in a specific context, but wants to limit the risk of it being stored or forwarded for use by other parties or in other contexts.

It's not even DRM, just a Win32 call that hides the window from screen capture. There's no anti-tamper or protected media path.

I don't think Signal is in the Windows app store, is it? I've never installed it from there.

And this isn't really abusing anything. It's just a flag a window can set to say it's sensitive and shouldn't be included in screen grabs.

it's good that microsoft isn't the arbiter of what should be allowed to be installed onto a machine.

I actively dislike walled gardens. It's fine to have a store, and to sell via a store, but it should never be an exclusive option.

Signal isn't in a Windows app store to be kicked out of, though