Brave search uses it. From my limited understanding, it sends a time-consuming javascript function and its input to your browser, and has your browser calculate the output and send it back. The server matches your output with the expected output. I assume the server would pre-compute in some way? On the spectrum, it leans more towards being a spam-alleviating thing rather than a human-distinguishing thing.
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> pre-compute
Or it could be a SAT or something that's easy to verify and hard to solve.
id think its some kind of proof of sequential work, basically an un-parallelizable calculation that is guaranteed to take a certain number of steps, and making solving thousands of them much harder and hopefully not worth it