[zozbot234]

Typically, "slower" updates in Debian testing are due to the fact that the new package version has known breakage that makes it unsuitable for release, or requires dependencies to be updated and this in turn breaks existing packages (either the build, or the package itself). These situations have to be managed via a "transitions" mechanism. When in doubt, there is a tracker at https://qa.debian.org/excuses.php that reports on why a newer version of the package has yet to enter the testing channel.

[shmerl]

Usually, but it's not the case here. In the case you described, you'll see the new upload in unstable, that didn't get to testing yet. In this case, it's simply not there at all:

https://tracker.debian.org/pkg/plasma-desktop

So I wonder if there is some kind of other blocker.

[zozbot234]

Looks like it's being kept out of unstable until other transitions are complete. E.g. see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=919218

[shmerl]

Interesting. Hopefully once this is resolved, new Plasma will follow.

And regarding KDE and Qt. I wish Qt already would have Vulkan options instead of OpenGL, and KWin would use it as well.