The scheduled-player package (specifically, the scroller plugin) makes use of some api.screen.set_scissor() features.
Is there any documentation on how this works? I’m currently working on implementing some scrolling text for another plugin, so it would be nice to not need to handle all the “determining how to hide the ‘outside scrolling area’ text” logic myself …
Found it! In the end, it’s using sys.get_ext "scissors", used in placement.lua line 78-93 
Nonetheless i would appreciate it if there would be some documentation about the available extensions and how to use them.
These extensions are currently intentionally undocumented as they are all considered experimental. The scissors for example behave weird (IIRC) if the native resolution and the one set by gl.setup don’t match. Also the current API feels bad as you can’t stack scissor calls. So that also needs some thinking before there’s an official API to use this feature :-/