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.