A lot of the implied_x tags are sort of problematic, but implied anal absolutely takes the cake. It was initially made on post #1722582 (and others in that pool), for cases where you couldn't be sure if it was anal or not (e.g. could be double vaginal or no penetration).
At some point it was completely co-opted by yaoi posters, and now if you look through it the majority of the tag is either implied_sex + yaoi or people taking it to an absurdly over-literal level and tagging it on any post where you can't literally see the anus being penetrated. Just look at some of the shit people have been tagging it on:
I'm reluctant to say it should be completely nuked because it seems it could be useful, but in its current form it's basically worthless. My suggestion would be to exclude it from most yaoi posts (and I guess futa on male or pegging) since that just overlaps with implied_sex. Thoughts?
Alternatively we could get rid of it completely and stick with only implied_sex/etc and just tag anal as relevant.