I opened Issue 23498 about exposing split_header_words() or similar. So this issue can focus on moving parse_header() to an email.policy.HTTP method or whatever.
RTSP 1.0 and its Transport header is defined in RFC 2326: <https://tools.ietf.org/html/rfc2326#section-12.39>. However it makes more sense to me to provide the generic header value parsing routines where possible, like parse_header() and Message.get_param/s(), rather than hard-coding them for specific header names (my vague understanding of what the header registry module does).
The python-dev post mentioned above seems to be at <https://mail.python.org/pipermail/python-dev/2009-April/088096.html>, with one response.