How to remove handlebar end plug (probably expansion type)?
I went ahead and removed the end where the screw was already out forcefully, meaning putting two screws into the side walls of the plug and levering on them with pliers. Once the plug reached out of the handlebar, I couldn't lever anymore and just pulling didn't do the trick -> abused a wood drill bit to mill away enough material that pulling worked. Weirdly enough, there wasn't any clear signs of the device being "abnormally" stuck.

The principle is simple: A split shaft is extended by pulling in a cone like counterpiece to the screw. To undo it, you need to untighten the screw, then push it in to release the cone. The cone has at two points protroding features, which when not aligned with the slits in the shaft prevent it from extending the shaft. So if slightly turned, you can pull on the end cap to retrieve the entire thing. You can use the loosened grip to have a better handle while pulling. If, as in my case, that isn't enough force, use a flat plier to go in between handlebar/grip and end cap and lever the entire thing out.

TLDR:
If you have this end cap, replace it asap (or regularly remove it, maybe that helps). If you have any other cap with a screw in the middle (likely expansion type), I'd advice testing to remove it and if it works fine, hope it will work fine again once you need to remove your grips. If it doesn't work fine, replace it (it will be trash anyway).