Dear Maintainer,
I added the splash parameter to GRUB with only the "nouveau" driver for Nvidia graphics card and plymouth seemed to be able to work. I say seemed because I couldn't get it to quite work but because I sometimes saw splash screens
for fractions of a second at boot and it didn't prevent the boot from completing, I'm pretty sure it's able to work with a little bit of tinkering. But after I installed the Nvidia proprietary drivers version 550, the boot
process just freezes if I add the splash parameter to GRUB. By that, I mean the splash screen isn't shown and I only see a white horizontal line on the top left of my black screen (probably some kind of cursor) start to blink
before staying lit after a second or two. It then stays in this position until I decide to forcefully shut down my PC by pressing down the power button for quite some time. If I boot without the splash parameter, the machine
boots normaly.
I tried replicating the issue after the boot in a KDE on X11 environment and using the following commands:
sudo plymouthd
sudo plymouth --show-splash
I was able to display the splash screen with the nouveau driver but not with the nvidia-drivers where it showed a blank screen (black screen on an LCD screen so it's grayish).
Maybe this is a problem with the nvidia proprietary drivers specifically in which case you may not be the right person solve this. Anyway, I'm sure you've got way more experience than I do with the Debian operating system and
Plymouth so I believe this is where I should report this.