The self moderation tag is removed permanently if the thread is moved to a board that does not support it, a reasonable explanation for this would be that members that commented on the thread while it was in Meta and not self moderated did not agree to be moderated by a forum member and as such, should not have to be. So, the same way posts that were started without self moderation being enabled cannot be changed to include it later on, as then the op would be able to moderate members that did not agree to that.