Most notable to the Flatpak 1.10 stable release is a new and more efficient repository format . This new format - which is already supported by Flathub - allows isolating metadata based on the CPU architecture used by the client, supports delta-based incremental updates, and other optimizations. In many cases the new repo format means no longer having to download megabytes each time but can be as little as ~20k updates. Thus it's a win for lowered bandwidth usage while also helping expand Flathub in supporting more CPU architectures thanks to the new design.
Flatpak 1.10 also has security updates and a variety of other enhancements as outlined via the release announcement .
For those not yet ready to move to the new 1.10 stable series, Flatpak 1.8.5 was released today as well that back-ports fixes whereby a malicious application could execute code outside of the sandbox.