After years and years of waiting, it's finally here! The PoW implementation was merged into the main branch of Tor software package on April 10, 2023! It's now only a matter of time for it to get in effect, the expectations are a few months. First Tor needs to push an updated version of their software, and then relay operators will need to catch up.
It will only then be revealed what the results are and if it'll help relief onion services and the Tor network, both suffering from massive DDOS attacks.
Proof of Work is usually used among cryptocurrencies for mining. It requires computers to use more power to solve a "puzzle", this in return deters most bots and spammers.
The Tor Project after receiving millions of dollars: