In 2015, e621's Uploading Guidelines were updated with the objective of restricting what can be approved as a post in the site. Content that was previously allowed, such as photos of sculptures, for example (see topic #29373), is no longer welcome since the change. However, posts that were approved before 2015 were grandfathered in. In fact, the only uploads that can get away with disregarding the UG today are superior versions/replacements of grandfathered posts.

For that and various other reasons, I felt that it was necessary to have a tag for such content. Thus, grandfathered_content was born. The problem is that, as you can see, the wiki definition is quite vague. I was unable to find much documentation addressing the 2015 change, and have been populating the tag with only the most obvious examples.

This topic is a request for help on creating a more detailed definition, as well as finding out what kind of post the tag should apply to. It is not as simple as it may seem at first: for example, human_only is frequently cited as (and deleted for) being considered irrelevant to the site, even though it is allowed today with certain caveats that are not extremely well defined either, such as the presence of tentacles.

Hopefully we can use this opportunity to outline all relevant changes and update the Uploading Guidelines page with much needed information.