I would think twice before adding those type of features to Anubis. I like how Anubis currently does one thing and do that well. Once you make a captcha-like service that also does useful work, users will eventually perceive it as a useful-work-service that happens to have captcha-like function on the side, and that new perception will get a lot more people upset about Anubis.
We see this with Recaptcha where when it was first launched, some news sites praised it as making good use of what would have otherwise been wasted human effort. But eventually I started to see negative comments along the lines of how Recaptcha is just extracting free work to train self driving cars, nevermind the part about stopping bots. Since Recaptcha is now sometimes non-interactive, I am not sure if that data is still used for training, other than to improve Recaptcha itself, but the negative sentiment still holds whether that data is used or not.
I agree, also making it do useful work could detract from how well it works as bot-prevention. Recaptcha suffered from its own success eventually, having to ask people to read nearly impossible things, and now many sites are instead asking me to solve puzzles with no practical use.
P.S. Great to see you, omoikane
The original reCAPTCHA which was used to train OCR came out back when Google was at least pretending to not be evil, hence the favourable coverage about old books. Now that the challenges are used to train Waymo cars (citation needed, but obviously they won't be sharing the data), and Google is definitely not tracking everyone with it (according to... Google, the adtech company), there's no positive spin you could possibly put on it.
Were Anubis to add crypto mining, even if all the revenue went to Techaro, you could still say "the enshittification is a shame, but at least they're not Google". Using the compute for BOINC protein folding somehow should be unobjectionable.