What's wrong after uninstalling
Removing Plymouth also removes the display manager, e.g.
gdm and a little bit more. Here is a snippet of the remove command on my system:
sudo apt-get remove --simulate plymouth
The following packages will be REMOVED:
cryptsetup gdm lightdm mountall plymouth plymouth-label plymouth-theme-ubuntu-gnome-logo plymouth-theme-ubuntu-gnome-text plymouth-theme-ubuntu-logo plymouth-theme-ubuntu-text ubuntu-desktop ubuntu-gnome-desktop unity unity-greeter unity-tweak-tool upstart
That's the reason, why you don't have a login screen anymore.
An advice to revert the mistake
You can always login on tty1 and reinstall
gdm and the other removed packages like
upstart. An overview of the removed packages can you find in
/var/log/dpkg.log. In my example I would have to reinstall
sudo apt-get install gdm lightdm ubuntu-desktop upstart ubuntu-gnome-desktop unity unity-tweak-tool
After that start your display manager, e.g.
sudo service gdm start
sudo systemctl start gdm
What is Plymouth?
Plymouth is an application that runs very early in the boot process
(even before the root filesystem is mounted!) and provides a
graphical boot animation while the boot process happens in the
apt-cache show plymouth
… and Plymouth replaces
Find more about Plymouth here.