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There is no reason to move this to the email package. email can already parse headers just fine. It might be useful to have a parse_header utility method, though, since currently the easiest way to parse a single header using the email package is:
 >>> from email.policy import HTTP
 >>> from email.parser import Parser
 >>> m = Parser(policy=HTTP).parsestr('Content-Type: text/plain; filename="foo"\n\n')
 >>> m['Content-Type'].content_type
 'text/plain'
 >>> m['Content-type'].params
 mappingproxy({'filename': 'foo'})
Which isn't as straightforward as the parse_header API when you are only interested in a single header.
It might also be useful to have the email MIME headers grow a 'value' attribute to return whatever the value is (in this case, text/plain), so it can be accessed regardless of the header type.
I would make parse_header be a utility method of the policy, I think. Then the email API would be:
 from email.policy import HTTP
 h = HTTP.parse_header('Content-Type: ...')
 value, params = h.value, h.params
It would then be trivial to implement the backward compatibility shim for the CGI parse_header method using the above.