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.
>That's odd, because in my experience almost all games run best in "Borderless windowed" mode.
I think the difference is between when you tell the game to run that way in it's settings, versus abruptly forcing it by doing alt-tab. Games where their default 'full screen' is really 'borderless windowed' don't have problems when you alt-tab, but a lot of them act weird when their 'full screen' actually is a true full screen mode and you force them out of it by alt-tabbing.
You get full-screen performance with the ability to easily alt-tab without mode changes.