20 degrees is just a rule of thumb most ACs have a specific temperature change they're designed to do. You can go past it, that's just what the intented to do and it might not work as well or be able to do it. Fwiw I'd always heard 30 degrees farenheight for most window units. Had an hvac guy explain it to me years ago but fucked if I remember how it works

E: not sure why I'm getting downvotes this is like a very common thing. Google it https://frederickair.com/home-comfort/reduce-the-stress-on-your-ac-with-the-20-degree-rule/