The sites that will be broken are disproportionately important though. Congress.gov/govinfo.gov, weather.gov, europa.gov, plus dozens of sites for libraries, and universities.

Looking only at how many sites use a feature gives you an incomplete view. If a feature were only used by Wikipedia, it'd still be inappropriate to deprecate it with a breaking change and a short (1yr) migration window. You work with the important users to retire it and then start pulling the plug publicly to notify everyone you might have missed.