The respective Update announcement threads for each branch should normally be your first port of call for that type of information, otherwise (as is often the case) your guess is as good as ours.
You can, however, obtain a list of packages from any specific Manjaro ISO, if that helps. Find a link to any ISO on the Downloads page and add .pkgs to the end of the URL;
Example, using a KDE Plasma edition ISO:
https://download.manjaro.org/kde/25.0.10/manjaro-kde-25.0.10-251013-linux612.iso.pkgs
Copy/paste and follow the URL to see the results.
A reminder that Manjaro produces three (official) editions, and two of those (namely Gnome and Plasma) are making some rather huge transitions which will likely not be a walk in the park for all distributions.
With a comparatively small Team I’m sure Manjaro (proper) is doing it’s best to have all editions released in unison. With respect the Stable branch, sometimes it has to “lag behind” to ensure all editions are delivered in a similar state of readiness.
The branch “closest” to Arch Linux (Stable branch) is Manjaro (Unstable branch).
Remember too that despite Manjaro being based on Arch Linux, it is not Arch Linux. Many packages are inherited directly from Arch, many are not.
Due to the nature of the branch, some things may or may not work, as they are still being tested, re-built and re-tested, verified – as many times as it must take – a process wherein the only predictability is that “the thing” will eventually (or sooner) be ready for general availability in the Stable branch.
I use Stable myself, but also keep a VM tuned to Testing branch, and from my experience downsides are rare. I never have a problem waiting for Stable to “catch up”, if all that’s needed is some patience.
However, I will point out that one of the prime motivations for switching to Testing (or Unstable) is generally to help out, where possible, even in small ways.
Edit:-
Another reason to switch to Unstable is for those who utilise the Arch User Repo (AUR).
Applications acquired via the AUR are fundamentally designed for use on Arch Linux – the Manjaro Stable branch is (as mentioned) is the closest to Arch Stable – it is therefore suggested for AUR users to switch to the Manjaro Unstable branch.
Regards.