The shell's currentOp() helper has some logic to fallback to the old currentOp command rather than the $currentOp aggregation stage when it is connected to the Atlas Proxy:

https://github.com/mongodb/mongo/blob/3a49875934cfd7e70d789b4ad7093286d189acc9/src/mongo/shell/db.js#L810-L815

It's possible that this fallback logic is safe to delete, though we should check with the knowledgeable folks on the Cloud team. My thinking is that since new versions of the shell are no longer shipped to users, this fallback logic should be ok to get rid of. We don't need it in the server's testing environment (because we don't test the Atlas Proxy in this environment), and the current remaining purpose of the mongo shell is to act as the testing harness for the server.