In the original invalidation thread for armpits tag: https://e621.net/forum_topics/17431
Some stats:
6% (6461 / 107979) of posts for armpit contain ~armpit_fetish ~armpit_lick ~armpit_hair ~armpit_sniffing
9% (9765 / 107979) of posts for armpit contain ~hands_behind_head ~arms_above_head ~raised_arm (keep in mind, poses tend to be undertagged)
This was used as justification to invalidate the tag - which was an obvious statistical fallacy associated with the claim that "armpit" was too broad.
It instead proves that "~armpit_fetish ~armpit_lick ~armpit_hair ~armpit_sniffing" and "~hands_behind_head ~arms_above_head ~raised_arm" are too narrow
For the majority of posts tagged with armpit, it was a relevant tag in that it demarcated a highly visible portion of a character.
Much like other parts of the body that are tagged, it's an easy add that aids in finding specific content and poses.
I'm of the opinion that we should invalidate "breast" and "butt", since they are simply too broad. Every character has a butt!