Comment 11 for bug 1928200

I still have this problem, but I have solved it.

I create files

file: /etc/systemd/system/kill-plymouthd.service
[Unit]
Description=kill plymouthd
After=umount.target
DefaultDependencies=false

[Service] Type=oneshot
ExecStart=/usr/bin/pkill plymouthd

[Install] WantedBy=final.target

file: /etc/systemd/system/rm-nvidia.service
[Unit]
Description=rm nvidia modules
After=umount.target
DefaultDependencies=false

[Service] Type=oneshot
ExecStart=/usr/sbin/rmmod nvidia_uvm
ExecStart=/usr/sbin/rmmod nvidia_drm
ExecStart=/usr/sbin/rmmod nvidia_modeset
ExecStart=/usr/sbin/rmmod nvidia
ExecStart=/usr/sbin/rmmod i2c_nvidia_gpu

[Install]
WantedBy=final.target

execute:
sudo systemctl daemon-reload
sudo systemctl enable rm-nvidia.service
sudo systemctl enable kill-plymouthd.service

for debug this problem i installed finalrd ane created executable

/etc/finalrd/sleep.finalrd

#!/bin/sh

set -x
if [ "$1" = "setup" ] then
    . /usr/share/initramfs-tools/hook-functions
    copy_exec /usr/bin/sleep
    copy_exec /usr/bin/mount

    exit 0
fi

#your debug script
mount
sleep 30

i have second system with ubuntu and in this second system i do not have nvidia drivers and i only created kill-plymouthd.service and this also work.

 and i did know why plymouthd is not killed befor umount root fs..