In the olden days, Direct3D9 let you bypass the window manager and present directly to the screen if you selected fullscreen, but didn’t promise not to crash if you alt tabbed out and in again. When computers got a little faster, and bypassing the window manager wasn’t as important, running games in borderless fullscreen windows became a worthwhile tradeoff to avoid the crashes. Direct3D10 and later promised not to crash when switching windows, but took a long time to be adopted as doing so meant you couldn’t sell your game to XP users.

With Direct3D12, things were changed so you could still get the minimum latency without traditional fullscreen mode, with its only remaining advantage being that you could set a different screen resolution. That was rarely a good idea, as it was usually better to scale the game internally than to let the monitor do it, and changing the resolution sometimes messes up desktop icon placement and makes the screen flicker for a few seconds when switching modes.

tl;dr: the main traditional advantages of both fullscreen and borderless windowed are now available in both modes, so there’s no particular reason to support both.

Do you not have to alt-tab when you want to pause a video on a secondary monitor when you are in full-screen mode? How do you get focus on the video player?

As far as performance goes, I don’t think there is much difference. And if they are only going to add full screen or borderless, borderless is the one most people are going to want.

I personally prefer borderless as tabbing in/out of the game is instant and leaves the view up, which is great if you need to take notes on something in the game.