Per systemctl lässt sich plymouth-start.service nicht deaktivieren.
root@MLD-Wohnzimmer:~# systemctl disable plymouth-start.service
The unit files have no installation config (WantedBy=, RequiredBy=, UpheldBy=,
Also=, or Alias= settings in the [Install] section, and DefaultInstance= for
template units). This means they are not meant to be enabled or disabled using systemctl.
Possible reasons for having these kinds of units are:
• A unit may be statically enabled by being symlinked from another unit's
.wants/, .requires/, or .upholds/ directory.
• A unit's purpose may be to act as a helper for some other unit which has
a requirement dependency on it.
• A unit may be started when needed via activation (socket, path, timer,
D-Bus, udev, scripted systemctl call, ...).
• In case of template units, the unit is meant to be enabled with some
instance name specified.
Wobei es mir auch lieber wäre, die Ursache zu beseitigen. Für meinen Vater ist es verwirrend, wenn da beim Booten so viel Text durchscrollt.