Indexed Mode prevents use of Layer Groups (and vice versa)
GIMP version: 2.10.20
Description of the bug
Images in Indexed Mode can't have Layer Groups, and images with Layer Groups can't be put into Indexed Mode. The options are grayed out. I don't see any reason why this feature shouldn't exist.
This isn't a new bug, and it's been indirectly reported before in issue #2700. That issue says gimp should notify users of the problem, this issue here is to report the problem directly.
Always reproducible. Simply try to apply layer groups and indexed mode to the same image.