I don't know if this is limited to Ubuntu 24.04 but deleting system users doesn't work. They disappear from the GUI but they stay on the server.

From the server logs I have a slighly different output for a couple different users which is also strange.

User 1:

Unknown option: f
deluser [--system] [--remove-home] [--remove-all-files] [--backup]
        [--backup-to dir] [--backup-suffix str] [--conf file]
        [--quiet] [--verbose] [--debug] user

  remove a normal user from the system

deluser --group [--system] [--only-if-empty] [--conf file] [--quiet]
        [--verbose] [--debug] group
delgroup [--system] [--only-if-empty] [--conf file] [--quiet]
         [--verbose] [--debug] group
  remove a group from the system

deluser [--conf file] [--quiet] [--verbose] [--debug] user group
  remove the user from a group

User 2:

Did not find a custom pool driver for user testing for PHP version 8.3, skipping.
Unknown option: f
deluser [--system] [--remove-home] [--remove-all-files] [--backup]
        [--backup-to dir] [--backup-suffix str] [--conf file]
        [--quiet] [--verbose] [--debug] user

  remove a normal user from the system

deluser --group [--system] [--only-if-empty] [--conf file] [--quiet]
        [--verbose] [--debug] group
delgroup [--system] [--only-if-empty] [--conf file] [--quiet]
         [--verbose] [--debug] group
  remove a group from the system

deluser [--conf file] [--quiet] [--verbose] [--debug] user group
  remove the user from a group

Doing it from the CLI works just fine so it must be a wrong Ploi command:

$ sudo deluser --remove-home testing
info: Looking for files to backup/remove ...
info: Removing files ...
info: Removing crontab ...
info: Removing user `testing' ...