You can use plasma styles and window decorations that have built in rounded corners out of the box instead. You can usually find them in the macos-like themes and Deepin, I think. Also, I think Klassy has editing options in it's window decorations to engage rounded corners if memory serves me.
KDE
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
Plasma 6 Bugs
If you encounter a bug, proceed to https://bugs.kde.org/, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.
Ah, so it is theme-dependent. Thanks for the reply.
Exactly. The downside of doing it this way, I don't believe any gnome/gtk apps will utilize the kde rounded corners from window decorations.
I think native rounded corners were introduced in one of the latest versions, Nicco Loves Linux did a video on it, I believe.
Hmm, it seems like he's made a video about it in 2022. I can't seem to find any newer video for Plasma 6
Hm, I thought he mentioned it in his 6.4 juicy video, but I can't find it either. Apologies, I must have confused it with something else.
I did manage to find this rounded corners proposal, and unfortunately it doesn't look like it'll be in kwin anytime soon.