The setting works in chromium in my testing.
You need to log-out for the setting to be applied, as systemsettings tells you.
Middle-click text pasting is very old and standard since forever. I did implement this setting WaylandServer: Add a setting to disable PrimarySelection (!2487) · Merge requests · Plasma / KWin · GitLab.
I didn’t know this was possible to disable this in firefox as it ignores compositor (https://www.reddit.com/r/firefox/comments/11by5gk/how_do_i_disable_the_highlight_middle_mouse/) but it has clipboard.autocopy about:config setting apparently.