I think it's only been with new DX12 presentation modes [1] that games can use "Borderless windowed" without input lag. Before that or without updated support, you would always get a noticeable amount of input lag when not using exclusive fullscreen. I played CSGO competitively for a while, and it was definitely enough to be annoying.
Not needing to care about win7/8 is relatively recent (a few years now, but a lot less than ten), and fully supporting both dx11 and dx12 with no caveats was sufficiently difficult that most games didn't really take full advantage of dx12 until they could drop dx11.
[1] https://learn.microsoft.com/en-us/windows/win32/direct3ddxgi...