> it doesn't seem like printing a more useful error message to them should be that hard.
I think the existing error message is useful:
$ pip search foo
ERROR: XMLRPC request failed [code: -32500]
RuntimeError: PyPI no longer supports 'pip search' (or XML-RPC search). Please use https://pypi.org/search (via a browser) instead. See https://warehouse.pypa.io/api-reference/xml-rpc.html#deprecated-methods for more information.
It says (1) what failed, (2) why it failed, and (3) links to a replacement, and (4) links to a deprecation explanation. That last link could maybe then link back to the pip issue or include some more context, but it's a far cry from being not helpful.