My steam updated the game automatically for me, which would have been a massive disaster, wasting hours and hours of making the game feel right. I had the game on do not auto-update in the steam options.

It seemingly ignored it completely. Is there nothing for me to stop this? I was lucky to had a back-up of the 1.8 files.

"Do not update automatically" only keeps the game from updating on Steam start but as soon as the game is started it will update. The only way to prevent this is to stay in offline mode which can't be done forever.