Hello everyone! I’ll have more details in when the F42 announcement but wanted to give you a heads up now so you can prepare if need be. Due to the adoption of composefs by default in Fedora 42 that upgrade is likely a one way trip.
That means once you upgrade to stable when it is released that you will NOT be able to rebase back to gts. However you can disable composefs, we do not recommend this unless you upgraded and found a critical problem and need to go back:
sudo rpm-ostree kargs --append='ostree.prepare-root.composefs=0'
Refer to the upstream documentation for more information: Composefs :: Fedora Docs
We’ll update the documentation with more warnings about rebasing, and we will be updating the rebase helper to ensure that you don’t put yourself in that situation. This means that casual rebasing isn’t going to be a thing this cycle, but once the cutover is complete in the fall we’ll reinvestigate.
How we played ourselves.
In our own small way we helped make this happen.
It’s an example on how a donation of an open source project can really accelerate stuff. We’re not special in that regard, this is what usually happens when cool stuff goes into the CNCF. We were just at the right place at the right time picking the right tech. 
Composefs is going to be everywhere (inside flatpaks too), and we have to get this transition done now and as quickly as possible.
Would composefs probably ended up everywhere anyway? Yeah probably, but at least you know it’s got a long term plan attached to it.