According to 116daef9b55d82a016d1fd3e3d877a2468c55d1a commit description
modules/msrp: Use "msrps://" URIs for the server when TLS is
loaded. TLS is mandatory for MSRP relays.
According [rfc4976#section-9.2](https://datatracker.ietf.org/doc/html/rfc4976#section-9.2)
9.2. Using TLS
TLS is used to authenticate relays to senders and to provide integrity and confidentiality for the headers being transported. MSRP clients and relays MUST implement TLS.
Yes, according to RTF proxy must support TLS, but this does not dictate the TLS usage on the client side. I think the client a free to use TCP only. I will check RFC restrictions for the client.