Here are the things I’ve gathered are non-cosmetic:
Based on https://kde.org/de/announcements/megarelease/6/, I think that many more things are not just purely cosmetic changes.
I must be missing something.
I would say you simply have the wrong expectation that with a so-called mega release a lot of big changes have to happen that the user has to notice immediately. However, many of the changes in Plasma 6 are probably not huge. But many smaller changes are also many changes.
And as others have already written, the changeover to QT 6 was probably not an easy task.
I also see Plasma 6.0 as a basis for things that are planned in Plasma 6.1 or later versions, for example, and are therefore not yet visible to the user. Articles on the development of Plasma are regularly published at https://pointieststick.com, which provide a more detailed insight into the development process.
On Arch Linux, Plasma 6.0.1 was released a few minutes ago in the testing package sources.
~~I am curious if this version will reach the official repositories or if there are still reasons to delay an official release.~~
Edit: That went faster than expected. Plasma 6.0.1 has been moved from Testing to Extra.