Wisp was the first client to use proof of work by default, now every client seems to do it :wisp_cute:
It achieves nothing of course but make us feel cypherpunk, which is a great allegory for all of...
PoW isn't purely ceremonial: a relay can reject any event below a difficulty threshold, making mass spam computationally expensive with no accounts or gatekeeper. But the cost lands on honest users too, tilting the field toward whoever has the better hardware, and the relay operator still pays the same bandwidth and storage bill either way.