A while ago I installed Kubuntu on top of Ubuntu because I had misunderstood that was an easy way to check it out. it was easy to install, just one apt package installed all that was needed, but it proved difficult to switch back to Ubuntu which I quickly wanted. Uninstalling the package left much Kubuntu stuff behind.
At the time I managed to do the important things, like switching settings back to defaults, getting the Ubuntu fonts, switching back the display manager. But several annoying things remained, like the startup logo in the splash screen still saying Kubuntu.
When you look on the internet there is much outdated information. It turns out that these startup/shutdown things are part of a "Plymouth theme" and the internet says to do update-alternatives on default.plymouth to switch to "ubuntu-logo", or to install the package "plymouth-theme-ubuntu-logo" in apt. But neither exist anymore. The current theme is "bgrt". But rather than switching to it manually, you can go back to the automatic default by:
sudo apt remove plymouth-theme-kubuntu-logoWay easier, since it updates all dependent things.