Still open issue https://bugzilla.mozilla.org/show_bug.cgi?id=1539685
You can watch YouTube video in mpv (just launch mpv with the link the video you want to watch) and, since mpv support HDR, you can watch HDR YouTube video on plasma 6. See this for how to set up mpv to play HDR video.
Linux apps support for HDR is still pretty minimal, unfortunately.
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.