Unprepared for proof-of-work extensibility
At the time the C-tor implementation of onion service proof of work came about, we didn't have a clear specification for how multiple potential PoW schemes would interact.
We're starting to specify that behavior more carefully, and it's now clear that the C-tor implementation will be insufficient no matter what.
C-tor currently requires that "pow-params" only appears at most once, that it always has 4 arguments, and that it can't contain a binary object.
We do expect that multiple pow-params lines will be necessary for servers that support multiple algorithms simultaneously. Those new schemes will likely need a different number of parameters, and we may want to allow binary objects if those parameter sets need to be too large for one line.
If we plan to deploy new PoW schemes during the remaining lifetime of C-tor, it needs to be updated to successfully ignore pow-params lines with schemes it doesn't recognize.
Related: