In https://ortc.org/wp-content/uploads/2016/03/ortc.html#idl-def-RTCRtpParameters:

muxId of type DOMString, defaulting to ""

[...] Note that including muxId in RTCRtpParameters rather than in RTCRtpEncodingParameters implies that if it is desired to send simulcast streams with different muxId values for each stream, then multiple RTCRtpSender objects are needed.

But I cannot find muxId within RTCRtpEncodingParameters... May be it means the encodingId field, but that is related to the RID header extension which, AFAIK, it's not the same as the MID header extension.