A big thanks to tevador from TorProject Proof of Work for .onion Services https://spec.torproject.org/hspow-spec/
The overall denial-of-service prevention strategies in Tor are described in the Denial-of-service prevention mechanisms in Tor document. This document describes one specific mitigation, the proof-of-work client puzzle for onion service introduction.
This was originally proposal 327, A First Take at PoW Over Introduction Circuits authored by George Kadianakis, Mike Perry, David Goulet, and tevador.
Awesome but what does this have to do with monero?
The code trevador used was adapted from Monero's PoW code.
Monero also gets a lot of use on Tor sites. Some people who like private money also are drawn to other private spaces online. So helping the Tor network beat the DDoS attacks it's been under is good for Monero.
Both are somewhat tangentially related to Monero.