never stopped POSTing, even though I configured nginx to always respond 403 to anything from them for about a year now.

Lol, there are definitely some stubborn stuff out there. I’ve been serving 418 to a bunch of SEO crawlers - with fail2ban configured to drop all packets from their IPs/CIDR ranges after some attemps - for a few months now. They keep coming at the same rate as soon as they get unbanned. I guess they keep sending requests into the void for the whole ban duration.

Using the 418 for undesirable requests instead of a more common status code (such as 403) lets me easily filter these blocks in fail2ban, which can help weed out a lot of noise in server logs.