The large white flags popping in on the right are quite annoying. They also reset, so every time I play the game they pop up again.

I couldn't care less about achievements.
If you know of a way to disable them I would appreciate it.

If there's no setting for it, you might be able to trick the game into thinking you're cheating: https://www.pcgamingwiki.com/wiki/Doom_64#Restoring_red_medical_crosses

Possibly using cheat commands within the game would have the same effect.

I don't know whether the same thing would work in Quake.

User jpeppe worked it out: https://www.gog.com/forum/quake_series/can_achievements_be_disabled

My computer does not recognise the filetype of 'achievements' (I am using Linux)
So, when I use a HEX editor to change it, it changes the filetype to TGA image.

The game does not recognise this file, so it recreates the old one. And I am back to square one.

I have tried different HEX editors, but the result is the same.
Is there a way to save it as the correct filetype on Linux?