The conclusion back then was that it's impossible to make a threshold that is both low enough and high enough.
You need some other mechanism that can distinguish bad traffic from good (even if imperfectly), and then adjust the threshold based on it. See, for instance, "Proof of Work can Work": https://sites.cs.ucsb.edu/~rich/class/cs293b-cloud/papers/lu...
Very crudely if you think that a request costs the server ~10ms of compute time and a phone is 30x slower then you'd need 300ms of client compute time to equal it which seems very reasonable.
The only problem is you would need a cryptocurrency that a) lets you verify tiny chunks of work, and b) can't be done faster than you can do it on a phone using other hardware, and c) lets a client mine money without being to actually spend it ("homomorphic mining"?).
I don't know if anything like that exists but it would be an interesting problem to solve.
It's possible that the services that reward users for running proxies (or are bundled with mobile apps with a notice buried in the license) would also start rewarding/hiding compute services as well. There's currently no money in it because proof-of-work is so rare, but if it changes, their strategy might too.
I still think it is possible with some customized variant of RandomX. The server could even make a bit of money by acting as a mining pool by forcing the clients to mine a certain block template. It's just that it would need to be installed as a browser plugin or something, it wouldn't be efficient running within a page.
Also the verification process for RandomX is still pretty intensive. so there is a high minimum bar for where it would be feasible.