I wasn’t keen to jetison my 1L SIS bottles in the Gran Fondo in Albi – bought especially to carry more liquid. But it looked pretty usual to discard and collect on the wheel, rather than stopping to refill. Sportives are more race and less ride though, and there are definite areas for dropping litter and bottles, where they are collected.
Frankly, you should leave nothing but tyre tracks. I never drop anything.