I assume people would just adapt to the culture by generally not ordering more than they will eat.
It is definitely rare, but not completely unheard of; more common if you're in a sitdown place that also does takeaway/delivery. I've done it a few times. One memorable incident was in Bradford where we arrived late at a curry house after interminable faffing around, were all extremely hungry, ordered more than we usually would, and halfway through the starters and giant naan realized that we'd overdone it. Think we got more than one meal out of the leftovers.
Portion sizes vary by restaurant, and also you’re not always the same level of hungry.
Just yesterday i tried a new restaurant, the meal was delicious and the portions were unexpectedly large (and i had a single dish, a single course).
I happily took the leftovers away.
No system is perfect and a culture of taking home leftovers does not necessarily reduce waste overall.
How do you adapt and order less than 1 thing?
Well not being able to take home leftovers does increase waste, because as others have pointed out there will always be cases where you will have leftovers, no matter how careful you are.
So the question as to which system results in less waste boils down to a question that must be answered experimentally.
Reducing waste on a societal level is complex. Cultural practices of restaurants boxing food to take away may result in less waste but it may also result in more waste.
I feel that having leftovers is never desiderable, with or without the ability to take them home. In France the restaurants are obligated to allow you to take leftovers home, and in my experience this has not changed anything on the behavior of people eating in restaurants. The only thing that changes is that in the rare case in which you have leftovers, you can take them home.
If they don't do half-plate and the plates are large I might ask around the table if someone wants to split or take a portion of mine.