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
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they do support systemd as an alternative), Slackware uses a BSD-style init system, PCLinuxOS uses the old-school (and no longer maintained) sysvinit, and some others still use runit or something else.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.