The amount of free space is a user setting; there is no hard coded “archaic” 10% rule.
However, this is likely due to poor permissions assuming significant free space exists
Also sonarr never just deletes anything in the media library, so that makes no sense. More than likely sonarr marks it as deleted as it no longer sees it.
0 free disk space = sonarr can’t determine the actual amount of free space = permission issue (in my experience). Could be on either end (source/destination).