iCloud Tabs no longer a feature in Safari 13?

A Reddit user claims Apple support said they won't sync until your Mac is running Catalina.

I noticed this problem a while ago, but didn't dig into it until now. I am using iOS 13.3 and trying to get tabs from 10.13.6.

The Apple support page clarifies what should work and what will not work:

iCloud Tabs works between devices running iOS 13 or iPadOS and macOS Mojave 10.14.4 or later, or between devices running iOS 12 or earlier and macOS Mojave 10.14.3 or earlier.

Apple's What's new in the updates for macOS Mojave page does not mention changes that would make things incompatible, but here is a much more detailed break down of the 10.14.4 update.

So:

Catalina & iOS 13 - iCloud Tabs should work

Mojave 10.14.4 & iOS 13 - iCloud Tabs should work

Any version of macOS less than Mojave 10.14.4 won't work without iOS 13 for iCloud Tabs.

If you're running iOS 12, then don't go above Mojave 10.14.3 or you will not have iCloud Tabs anymore.

I am in the exact same situation, two iOS 12 devices and a macOS 10.14.6 machine. When running Safari 13.0.2 on my MacBook Pro I cannot see the iCloud tabs of my iOS devices, but the iOS devices can see each other's tabs. Download the Safari Technology Preview, which is version 13.1.2, and I can confirm that the iCloud tabs work again as expected. I can see the Safari tabs that are open on my iOS tabs devices again on my Mac. We'll have to wait until Safari 13.1.2 becomes available.

I solved it on my side now so I will propose my solution. I used to beta test macOS Catalina on an external SSD connected via T3 to my MacBook Pro. Naively, I logged in with my AppleID on that account and turned on all things iCloud.

For quite understandable (now) reasons the two logins from the same machine in two different macOSs created a conflict in the sync. I booted today from that disk, signed out from my AppleID, saving a copy of the (possibly) not uploaded things to a folder which then I emailed me, and then rebooted into my main disk with Mojave.

Headed into System Preferences > iCloud, turned Safari back on and, after a few minutes, everything started working again. Maybe you have a similar situation, or even another machine which is just logged in with the same AppleID.

I hope this sheds some light at least.