We have esrb - it has some questions on it that belong on that tag, we don't appear to have any questions raised for any of the other major rating bodies;
ACB, the Australian media rating system.
ESRB, the United States and Canadian computer and video game rating system
ELSPA, the former British computer and video game rating system, replaced by the PEGI system.
PEGI, the European computer and video game rating system
USK, the German computer and video game rating system
OFLC, the New Zealand media rating system
GRB, the South Korean computer and video game rating system.
Eirin, the Japanese film rating system.
We do have questions mentioning these bodies though. Would it be worthwhile creating a new tag to encompass all related questions to the correct tag (we have some questions on version-differences for example that would be better under a generic media-ratings tag) and then synonym the others to that so that in future any such questions get raised under this new tag?
I don't think it's worthwhile to create a new tag: rather, I think these should be handled like the platform tags:
If the question is about the ratings body, it should be tagged with the ratings body. Right now, we only need esrb1, as nobody seems to be interested in asking questions about the other bodies. If someone does ask a question about a different ratings body, we should create a tag for that ratings body then and there.
However, if a question merely mentions a ratings body and that mention is incidental to what the question wants to know (example), the question should be left untagged.
Note 1: to that end, I'm not sure why the DO NOT USE tag wiki was added to it: it's not currently part of the tag cleanup, and it doesn't run into any of the problem categories outlined by Oak. I've gone ahead and approved your tag wiki suggestions removing that notice.