Even if quiet is specified in GRUB_CMDLINE_LINUX_DEFAULT, I started getting this error in journald:
systemd Failed to start Splash screen shown during boot.
Operating System: Manjaro Linux
KDE Plasma Version: 5.27.7
KDE Frameworks Version: 5.109.0
Qt Version: 5.15.10
Kernel Version: 6.5.2-1-MANJARO (64-bit)
Graphics Platform: Wayland
Did you specify quiet AND nosplash?
I never set nosplash argument because Manjaro by default has its own splash disabled.
This message was not appearing in journald, it started to happen recently with kernel 6.5.
No one can help here because your clear information is missing.
Which splash screen tool do you use for booting?
There are two different tools:
If you remove quiet in GRUB_CMDLINE_LINUX_DEFAULT, is this error message gone?
Systemd can tell you which service name displayed this message. But without your info we do not know which service name.
Which splash screen tool do you use for booting?
I don’t use any customized splash, my installation is not changed, it uses Manjaro default settings.
I only asked why this error started to appear with kernel 6.5.
Find which systemd-unit failed:
$ sudo systemctl list-units --failed
If it displays a failed service, then run:
$ journalctl -u [SERVICE NAME].service -b
Check if splash boot was installed
$ pacman -Qq | grep splash
sudo systemctl list-units --failed
[sudo] password for medin:
UNIT LOAD ACTIVE SUB DESCRIPTION
0 loaded units listed.
It shows nothing.
pacman -Qq | grep -i plymouth
… maybe?
It also returns nothing.
I guess, systemd-boot has the feature splash screen. But it is not enabled by default.
$ ls -al /usr/share/systemd/bootctl/ | grep splash
The output:
.rw-r--r-- root root 157 KB 2 days ago splash-manjaro.bmp
You are using GRUB, try to disable systemd-boot.
I think Manjaro uses by default Systemd Boot, even Plasma desktop uses it for starting all of its services.
No, the default boot loader is GRUB.
I think you are getting a few things mixed up here. systemd-boot is a boot loader, as an alternative to grub.
What you are referring to is systemd, which is a system daemon and service manager. systemd-boot is a project developed by the same people who develop systemd, but it doesn’t have anything to do with systemd proper.
In Manjaro, Plasma does indeed build upon systemd, but that’s unrelated to Plasma itself. Plasma also runs just as well in distributions that do not use systemd. Gentoo for instance uses its own openrc by default (although they do support systemd as an alternative), Slackware uses a BSD-style init system, PCLinuxOS uses the old-school (and no longer maintained) sysvinit, and some others still use runit or something else.
Plasma itself is not tied to any particular init system — unlike GNOME, which does have systemd as a hard dependency, courtesy of it being developed at RedHat, where systemd is being developed as well.
It was systemd lead developer Lennart Poettering who, in his (notorious) narcissism, requested that the GNOME developers would make systemd a dependency of the GNOME display manager gdm. KDE on the other hand has no ties with RedHat — on the contrary: RedHat has for many years boycotted KDE and is still quite hostile to KDE. They’ve even stopped supporting it (again) in Fedora.
At least on xfce the default installed splash is Plymouth
I removed Plymouth package and it solved the problem for me. I’m using XFCE desktop environment.
Well, I never installed or enabled that systemd-boot.
plymouth package is already uninstalled on my laptop.
We are the Manjaro community, ChatGPT. 
quiet doesn’t prevent from showing boot error.
As a preventive measure I would remove the account. No need for people doing ChatGPT queries and posting it here, especially as a first post.
We do have rules about people using ChatGPT. But of course, this would require people to actually read the rules. 