`"sctp-transport"` stats were removed in https://github.com/w3c/webrtc-stats/pull/627 for lack of implementation (they now exist [here](https://w3c.github.io/webrtc-provisional-stats/#RTCSctpTransportStats-dict*)). This is probably why the ICE ones remain mandatory, because [other browsers](https://wpt.fyi/results/webrtc/RTCPeerConnection-mandatory-getStats.https.html) implement them.
Bug 1542938 Comment 9 Edit History
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`"sctp-transport"` stats were removed in https://github.com/w3c/webrtc-stats/pull/627 for lack of implementation (they now exist [here](https://w3c.github.io/webrtc-provisional-stats/#RTCSctpTransportStats-dict*)). This is probably why the ICE ones remain mandatory, because [other browsers](https://wpt.fyi/results/webrtc/RTCPeerConnection-mandatory-getStats.https.html) implement them, and there's no other API for RTT.