Alt text:
Twitter post by Daniel Feldman (@d_feldman): Linux is the only major operating system to support diagonal mode (credit [Twitter] @xssfox). Image shows an untrawide monitor rotated about 45 degrees, with a horizontal IDE window taking up a bottom triangle. A web browser and settings menu above it are organized creating a window shape almost like a stepped pyramid.
Edit: alt text
Rotating the display by a custom angle is possible through xrandr on X.org.
There's no Wayland protocol for custom angle rotation, and I don't expect anyone to create a protocol extension without a use-case.
My wild guess: Theoretically it should be possible for a compositor to support similar custom rotation, as applications simply draw to their surface (window), without knowing how and where it is displayed on the viewport (display).
But it might require quite a bit of work, depending on the project, so I don't expect to ever see custom rotation on anything besides smaller/niche compositors.
[1] https://unix.stackexchange.com/questions/552138/rotate-a-display-by-custom-angle#552140
Puh-lease. It's Wayland; the devs fully and honestly expect every app developer (eg.: calc, Libreoffice, notepad.exe) to implement custom angle rotation on their own.