Please read up on RandomX and the history of Equi-X: https://github.com/tevador/equix/blob/master/devlog.md

Both generate "random" code based on the hash of the last block and you have millions of algorithms which you simply can't put onto silicon, since actually a modern CPU/GPU is simply the best ASIC. RandomX has been out four years now and hasn't been "broken" by any kind of ASIC.

Sure, an old CPU will take x10 or x100 the time to solve, but this is acceptable to mitigate DoS, when an attacker needs 1 second and someone with a 15 year old Pentium needs 100 seconds. Captchas used on hidden services take mostly more with several failures but don't protect the service like this solution does.