They have never been there - at least I never saw them - so they cannot be missing.
You should know that this kcm applet is simply listing the available Plymouth themes by name.
The kcm applet acts as a frontend for plymouth-set-default-theme which has no option for preview.
$ plymouth-set-default-theme --help
Plymouth theme chooser
usage: plymouth-set-default-theme { --list | --reset [ --rebuild-initrd ] | <theme-name> [ --rebuild-initrd ] | --rebuild-initrd | --help }
-h, --help Show this help message
-l, --list Show available themes
-r. --reset Reset to default theme
-R, --rebuild-initrd Rebuild initrd (necessary after changing theme)
<theme-name> Name of new theme to use (see --list for available themes)
Previewing a theme is possible but only manually using a TTY as described at Plymouth - ArchWiki