so im seeing some commentary in tags about making tagging etiquette for crossovers, and im here to deliver amazing news! there IS tagging etiquette for crossovers! it just simply isn't being utilized.
on a fanfiction site, such as ao3, proper etiquette is to tag for all properties included. this lets people who aren't interested specifically filter out crossovers.
on tumblr dot com, where filtering out tags is not possible (no, blocking a tag does not count), tagging a crossover post with all root properties is INCREDIBLY rude, specifically for this reason. no one likes seeing their tag overrun with things that they aren't interested in.
there are rare exceptions! if it's a one off post or particularly rare crossover, it's fine to main tag. but as soon as something gains a lot of traction, like has happened here, main tagging will literally run people out of their own property tag.
(as an aside, there are accepted exceptions for Canonical Crossovers-- stuff like deadpool vs wolverine. but most of those take place in the Comics Fandom which has a very different approach to this stuff anyways, since so much of it is already crossovers at a source material level)
however, the correct etiquette is to tag for the crossover (phm x il / project hail mary x iron lung / any iteration of that) or ship name (bloodymary), and the individual relevant characters (grace phm, simon iron lung, etc etc). this allows those interested to locate the posts, while preventing the main tags from being inundated and becoming useless.
crossover tagging etiquette is also dependent on relevance. i.e.: if you have a crossover with three properties, but only two of them are the focus and one plays a bit role, you only tag for the two prominent ones. a real life example: i have posted a kingdom hearts fanfic to ao3 that is technically a crossover, because it briefly mentions two naruto characters by name. it is not tagged for naruto, as a crossover, or for those characters, because by doing so i would be functionally submitting spam (stupid, pointless, annoying message) to those tags. i would be making it harder for people who want to find a naruto fic with those characters to actually do so. and it would also waste the time of anyone looking to read a real crossover.
that's rude. it's rude because fandom is a communal activity, and we do actually have a responsibility to keep it nice for those around us. that means we have to be cognizant on how our actions, tags, and posts will effect the greater fandom ecosystem. this is especially true for crossovers.
so in this scenario, where 90% of bloodymary tumblr posts and premises take place in phm, it should not be tagged for iron lung at all.
there are rules. they do exist.
i know it feels like Having Fun With Blorbos shouldn't have rules, but they exist to keep the peace and make sure everyone can have a good time with blorbos, not just you.