For example in the Image Editor, an assert would be triggered after
unlinking an image [with setting users to zero] and then setting the
image for the Image Editor again.

Whenever we set an Image for Image Editor, the Image ID is flagged
LIB_TAG_EXTRAUSER_SET, when we unlink [with setting users to zero] this
flag was not cleared.

quote @Bastien Montagne (mont29): a proper fix would be to move this to modern code, and
actually delete the ID... but that is for later.

Being unsure about all consequences, I would not consider this safe for
2.83 release though...