https://github.com/w3c/webrtc-pc/pull/2223 added a closing event to RTCDataChannel. Tests are being updated in https://github.com/web-platform-tests/wpt/pull/21117.
<rdar://problem/96303438>
While we seem to have stubs for this -- I see that we fail the WPT.fyi tests.
We pass all of https://wpt.fyi/results/webrtc/RTCDataChannel-close.html?label=experimental&label=master&aligned so I think we are good now.