Inroduction:
After having installed Linux Mint 19.3 on an old MacBook Pro I wanted to be able to use the function keys without having to press on the ‘Fn’ key
Here is the answer:
Reference: https://www.reddit.com/r/linuxmint/comments/4cecsz/set_f1f12_as_default_function_without_pressing_fn/

Solution:
You can run a simple command to change the behavior.

echo 0 > /sys/module/hid_apple/parameters/fnmode
    0 = Fn key disabled
    1 = Fn key pressed by default
    2 = Fn key released by default

Therefore to get what I wanted the command is:

echo 2 > /sys/module/hid_apple/parameters/fnmode

To make it survive a reboot you just need to add the following to /etc/modprobe.d/hid_apple.conf:

echo "options hid_apple fnmode=2" >> /etc/modprobe.d/hid_apple.conf