I see there are 52 questions tagged all-sites, which seems used for saying that the issue is for all the sites.
Is there really any difference if the question is about two/three sites, or it is about all the sites? Since all the sites but Area 51 use the same base code, if the bug is not an issue in the CSS styles, then it is probable the bug is present in all the Stack Exchange sites. Just in the case the code is being tested on this very site, the bug is not visible on other sites.
The tag has been first used on a question dated 2009, and there are other questions using that tag dated 2010, 2011, and 2012. It is not a tag that has been recently introduced.