Fix mouse acceleration appearing on replug of a mouse

TL;DR

Create /etc/X11/xorg.conf.d/50-mouse-acceleration.conf

Put the following in it:

Section "InputClass"
    Identifier "My Mouse"
    Driver "libinput"
    MatchIsPointer "yes"
    Option "AccelProfile" "flat"
EndSection

Explanation

On some systems, when you unplug and replug a mouse, mouse acceleration is re-enabled

What can fix this is to create a file in /etc/X11/xorg.conf.d/(the name doesn't matter, but it must end in .conf. I chose 50-mouse-acceleration.conf for clarity)

In this file, put the following:

Section "InputClass"
    Identifier "My Mouse"
    Driver "libinput"
    MatchIsPointer "yes"
    Option "AccelProfile" "flat"
EndSection