This ends up changing the behavior of bug 243294, but https://wpt.fyi/results/webrtc/RTCDataChannel-binaryType.window.html suggests that wasn't very interoperable to begin with. And this would align it with WebSocket, which seems good.
<rdar://problem/114559008>
Pull request: https://github.com/WebKit/WebKit/pull/17121
Committed 267353@main (6cabcd45af4d): <https://commits.webkit.org/267353@main> Reviewed commits have been landed. Closing PR #17121 and removing active labels.