It's not clear yet if it will be used in official documents after Brexit agreements will be signed, but UK was the only country who chose English as an official language.
There's an hard requirement in EU to translate every official document in every language spoken in the EU, if English is not an official language, that requirement will cease to exist and slowly there will be no official document in English anymore because it's also a very costly operation.
Unless Ireland or Malta apply for English as primary language.
It's also a political stand, Macron is pushing to use French more and more in the EU institutions.
It doesn't mean that it won't be taught in schools or that it will fade as the most spoken language in EU, it still is by a large margin, but that it will move away from UK English and will become more similar to the English 75% of the > 1 billion English speakers in the World speak as a second language.
Native English speakers not understanding or not being understood by non-native speakers it's already a thing, the gap could widen in the future.