Apple - iCloud Tabs no longer a feature in Safari 13?
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.
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.
A Reddit user claims Apple support said they won't sync until your Mac is running Catalina.
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.