When on the world settings via the main menu, you can change the default gamemode.
Changing it to creative mode gives you the prompt "Turn off achievements" confirming you want to do that by allowing you to "Continue" or "Go Back". However, at this point achievements are already disabled and cannot be re-enabled even by clicking "Go Back", making the prompt redundant and false.

This can be done by accident, which was how I did it, and as far as I'm aware it is irreversible.

Should also mention switching back to survival/disabling cheats also does not re-enable achievements.