As there has been no liftoff release for two month, I really hope there are still developers working on these upcoming changes. Liftoff is still my number 1 Lemmy app.
For those who prefer text, the video is from gamingonlinux and there is also a written review on their website.
The most immediate change you'll notice is that Valve tweaked the default display colour settings, as they said it has been adjusted to "emulate the sRGB color gamut, resulting in a slightly warmer and more vibrant color appearance".
On top of that for external displays the Steam Deck now supports HDR, and you can also enable VRR if supported by the USB-C adaptor you're using.
You're right, changes that break compatibility are unlikely. But they can happen and happened in the past (e.g. #3082)