2 phase drop is going to only do 2 phase drops when in featureCompatibilityVersion 3.6. The w:majority write during downgrade will make sure that all of those drops are committed before downgrading to 3.4 and prevent undropped collections from sticking around. There is currently a race between when the w:majority-write is written and when the featureCompatibilityVersion server parameter is set, such that we could start a new 2 phase drop in that window that would never get cleaned up because we downgrade before it's fully dropped.

It should be safe to set the server parameter first on downgrade and then write the w:majority document to ensure no new 2 phase drops are begun.