I'm on Rpi-4 and the OS is DietPi.
I have modified /boot/cmdline.txt using the following parameters to disable all and every boot-time logs:
console=tty3 root=PARTUUID=907af7d0-02 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait net.ifnames=0 logo.nologo consoleblank=0 loglevel=1 quiet vt.global_cursor_default=0 fastboot
It has worked and most of the logs are vanished but the following logs are still visible:
This is bootloader's log obviously. You can edit its config file using:
sudo -E rpi-eeprom-config --edit
DISABLE_HDMI=1 to disable the HDMI boot diagnostics display.
Rpi official link