Release notes:
Windows 7 and later
macOS 10.10 (Yosemite and later)
Ubuntu (16.04 Xenial) & (18.04 Bionic)
Linux Mint (18.x) & (19.x)
Debian (8 Jessie) , (9 Stretch) & (10 Buster)
Fedora (27) , (28) , (29) & (30)
Every new release I checked out if the insync-thunar (for Insync3) will be again available …
And again and again I realized, that Insync developers does not work on this plugin … So what???
Even with this new version I have problem. Some folder was not synchronized automatically. I had to run it manually. Also, some files were deleted. Fortunately, I have the backup to put them back. I am tired each time to check the synchronization and also if the files were not deleted. I am moving to 1.5 until everything is fixed.
Really sorry for the trouble! Our engineers are working to bring back the Thunar integration. 
We’ll keep you posted as soon as it’s ready!
I use Insync Ubuntu 18.04 repository, as was recommended in first 3.* release, i.e. with source line
deb http://apt.insync.io/ubuntu bionic non-free contrib
yet the 3.0.21 wasn’t in there. Shall I again install it manually from .deb every time?
Can you give it another go? We usually update our repos shortly after the early forums release. Let me know how it goes!
What I did:
[…no errors reported…]
insync | 3.0.20.40428-bionic | http://apt.insync.io/ubuntu bionic/non-free amd64 Packages
So far, no 3.0.21 …
Any idea of when XFCE’s Thunar is going to get some love again? Is it on the roadmap of Linux file manager integrations?