I am having the same problem. It is possibly tied to a Mac OS Safari extension, but the problem has since spread to my iOS devices, so I'm not convinced of that. More likely the problem simply began after I quit and restarted Safari on my Macs.
I am using Mac OS 11.1 and Safari 14.0.2. It started with my iMac. I have three other devices on the same iCloud account. It still worked on the other three initially, but curiously, although they showed tabs open on my iMac, they were all old tabs from my iMac—opened last night, not what I had open today when I noticed the problem. In the morning I had to restart Safari on my iMac to update an extension called Dark Mode for Safari. Can't recall for sure, but I believe those three tabs from last night were still open at the time, so they were probably the last signal sent to iCloud.
It still worked on my MacBook (same software versions), but after a couple minutes I got the popup to restart to update the same extension. After that, I could not see iCloud tabs on the MacBook. Right after that, it stopped working on my iPad but continued to work on my iPhone. My iPad needed to be updated to 14.3, so I did that and it did not fix the problem. (My iPhone was already on 14.3.)
I tried disabling the extension, and then I tried uninstalling it, but that didn't fix the problem.
The
instructions I found on Apple's help forum (a few others have recently complained of this problem) addressed fixing the issue for just one Mac and an iPhone. To streamline things, I simply powered down the MacBook and the iPad, and then followed the steps with just my iMac and iPhone. No luck. I even gave them each an hour to sync before checking to see if anything changed. I am stumped. I may try improvising the steps using all four devices (I still haven't powered on the MacBook or iPad), but I'm not entirely sure what the sequence should be. I guess I would toggle iCloud off and on on both Macs before both iDevices. I'm also considering turning off iCloud entirely on my devices, not just iCloud Safari, and then turning them back on.
But that's a lot of work for something that is annoying, not crippling. At this point mostly I am hoping this is a bug someone will fix soon.