Thanks to @nicman23 's contribution and a little ‘elbow grease’, I am trying to maintain the Locally Integrated Menu decoration originally written by @zren.
You can find it here: GitHub - guiodic/material-decoration: Material-ish window decoration theme for KWin, with LIM, based on zzag's original design.
Compared to the original version, this decoration works on Plasma 6 and also fixed the bug it had with Libreoffice whereby the menu would often disappear.
Those using Arch or derivatives can install it from the AUR (material-kwin-decoration-git).
Please note: Wayland is not supported.
Those who would like to help are welcome!