Some time recently, AO3 tag wranglers de-canonized the “Sherlock Holmes & Related Fandoms” metatag. This involved detaching the fandom subtags for almost 100 different Holmes-related adaptations, reboots, and spinoffs, and then making the “Related Fandoms” metatag into a synonym of “Sherlock (TV)“.
I don’t actually know when the switch was made (though it couldn’t have been more than 4 weeks ago), but within the past week, fandom-at-large (a) noticed, (b) was horrified, and (c) inundated AO3 Support with a flood of complaints.

The fallout is being discussed all over the place — here’s a nice roundup of some reaction links — and there’s a heck of a lot of misinformation swirling around. Gonna try to set some of it straight here.
Disclosure: I’m a wrangler. An average one, not a supervisor or a chair. I don’t wrangle any Holmes-related fandoms, and was not part of the Holmes &RF discussion. This is supposed to be a facts-and-info post; any personal opinions that do show up are my own, not speaking on behalf of any other wrangler(s).
There are some links to individual fics in here — they’re not supposed to represent “good tagging” or “bad tagging”, they’re just examples of “a kind of tagging somebody has done.” Please don’t be weird to the authors.