I have the same issues as most of your. iCloud tabs only syncing every couple of hours. History doesn't seem to sync at all. It was all working fine a few days ago and yeah happened after Mac OS 10.15.7 I have escalated the case with Apple support to the senior engineer level. They still say it's not their backend issue. I will keep your updated on how the troubleshooting goes later.
Anything further from Apple?
Has anyone else filed a bug report (and if so, under what product)?
Nothing has to have been changed. Between your iCloud data and your device is intermediary, regional server caches. If the caches are failing to update in a particular category, then iCloud will have your correct list of open tabs but one or the other of your devices will be syncing to an out of date cache. In such a case it will self-resolve once Apple becomes aware of it, without any "change" to iCloud or OS 14.Wonder what they changed that it's so broken.
I haven't seen any topic-related threads in apple.discussions...