Restaurant owners, employees, and other customers all have to abide by safety precautions to limit the spread of the coronavirus as the number of cases increases. But the rudest thing you can do at a restaurant doesn't have to do with wearing a mask or washing your hands, even though skipping out on those is incredibly dangerous.
The rudest thing you could do is making a reservation and then not showing up. Some restaurants require them in order to have an idea of how many people will be in the space. Knowing who is dining and when can also help with cleaning in between guests.