Moved to the Software & Applications section.
A quick:
- Code: Select all
apt-cache rdepends libdrm-nouveau1a
show that plymouth depends on this package. Running the same command for plymouth shows other packages depending on plymouth, and so on, and so on, probably to well over a hundred of your packages

So that explains why hundreds of packages will be removed if you remove libdrm-nouveau1a, but I can't answer why plymouth needs this package. Then again, this is a library with an installed size of 86 KiB, so perhaps don't spend much more time on it.