For some reason we don't implement startObservingCookieChanges() and stopObservingCookieChanges() for soup, and we set the observer callback from the network session constructor/destructor.
Created attachment 436873 [details] Patch
Created attachment 436874 [details] Patch for landing
Committed r281801 (241138@main): <https://commits.webkit.org/241138@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 436874 [details].
<rdar://problem/82578143>