The Russos sent out a similar ‘no spoilers’ request when Avengers: Infinity War came out, as four months earlier during test screenings, they realized that the impact of the movie was going to be more significant than they realized and wanted to nip this issue in the bud as much as possible.
Obviously it’s impossible to clamp down on spoilers 100%, but as Joe Russo explained to THR, he and his brother were just doing their best to ensure that etiquette was followed and that people were respectful to one another.