You say that but so far nothing points to it being developed for that device. It seems to mostly be a personal project by the developer, especially because this is the first time a bunch of KDE developers seem to see the code. The author of the MR even updated the MR to mention they do not work for Valve. That doesn't mean it isn't useful for that device of course, it very much makes sense to run on there. But again, the black and white cameras of that device makes this mode less interesting.
Or KDE's built-in Flatpak permissions settings. But yeah I guess, it's mostly needed for applications that haven't adopted to the new Portal API's yet which is the better solution, but this works for now until applications have updated.
Source? I found the MR, https://invent.kde.org/plasma/kwin/-/merge_requests/8671, and nothing about that shows it's for the Steam Frame. In fact, the video shows a colored passthrough camera mode while the Steam Frame (sadly) has a black and white camera only.
what a flatpac is and why people hate it,
Huh, most people actually like Flatpak, and for good reasons too.
Huh, is this something that's actively been developed? Is there a MR?
My Valve Index only has black and white cameras. And what other headset are you talking about? They never made more.