Edit service:

nano /etc/systemd/system/getty.target.wants/serial-getty@ttyS0.service

Find the line starting with ExecStart, make a copy and comment it out (as a backup / retreat path), then edit the active line. Remove –keep-baud and perhaps also leave just your one desired baud rate = reduce the comma-separated list to a single item. Like this:

#ExecStart=-/sbin/agetty -o '-p -- \\u' --keep-baud 115200,57600,19200,9600 %I $TERM
ExecStart=-/sbin/agetty -o '-p -- \\u' 115200 %I $TERM

Note: you have hereby edited a systemd unit file. You will need to tell the systemd superdaemon to reload it. (If you forget to do this, systemctl will suggest you to do it.)

systemctl daemon-reload

Time to start the already altered service unit:

systemctl start serial-getty@ttyS0
systemctl status serial-getty@ttyS0

To test if it works, use your favourite terminal emulator. An example how to start minicom:

minicom -b 115200 -D /dev/ttyS0

To enable screen while system is loading, need to enable it in GRUB bootloader:

nano /etc/default/grub

Edit or append lines:

GRUB_CMDLINE_LINUX='console=tty0 console=ttyS0,115200n8'
GRUB_TERMINAL=serial
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"

Then update Grub

update-grub

When connecting from another Linux, can use minicom

apt install minicom

Detect connection

ls /dev/tty*

For example, /dev/ttyUSB0

Then connect to remote PC

minicom -b 115200 -D /dev/ttyUSB0