OBS continued using the EOL runtime because of Qt regressions introduced in the updated KDE runtime. The OBS team decided the security risk of sticking to the EOL runtime was small, so they didn't update.

But that still does mean that users were no longer receiving security updates. Ideally, OBS should have moved to the standard Freedesktop runtime and vendored in the older Qt dependency. That way, the they would still be receiving security updates for everything in the Freedesktop runtime. Then once the regressions were fixed, they could move to the updated KDE runtime and remove the vendored Qt dependency.

Overall, the risk OBS had was small. But it demonstrates a larger issue with Flathub, which is that they don't take security as seriously as Fedora. There are hundreds of flatpaks in Flathub that haven't been updated in years, using EOL runtimes and vendored dependencies that get no updates.