Oh my mistake, sorry. This has been implemented but only for Plasma 6. It couldn’t be backported since it contains a new string. See Sync cursor size too (!41) · Merge requests · Plasma / SDDM KCM · GitLab
I’ve proposed backporting the backportable non-string-change parts: Sync cursor size too (!43) · Merge requests · Plasma / SDDM KCM · GitLab