Short update, to report that 15.0-U1.5 is out, and that the issue tracker is now discussing 15.1-RC2.
Dedication to keep this up: 
I promise I’m not trying to be a cheerleader for FreeCORE on the TrueNAS forums, but I want to point out something incredibly useful that would benefit TrueNAS: an online interactive demo. Try them out right now.
If you visit the link, you’ll see there are two interactive demos for 15.0 and 15.1-RC. Check out the demo for 15.1-RC.
Why am I posting about this on the TrueNAS forums? Some might remember this feature request I made last year.
How will this benefit TrueNAS? It allows new and existing users to explore the UI and experiment with how elements interact with each other without having to create a VM or play with their production system.
Over the week-end I have migrated three systems from TrueNAS 13.0-U6.8 through 13.3-U1.2 to FreeCORE 15.0-U1.7. All went smooth, and the backup system which pulls snaphots from the main at midnight just kept carrying on like nothing happened.
Very boring update. I 
it that way!
There is however one change to report: Copy-paste from the webshell works at it should!
This feature is reportedly added to 15.1-RC6:
Configuration
Save Configuration can write a file for moving to TrueNAS SCALE: the configuration in the form a TrueNAS SCALE installation accepts, with a README that lists each pool’s ZFS features and the TrueNAS release that can import the pool.
And the 15.0 branch moves to U2.0 (not to say U1.10?).