For the question to resolve yes, the number of Muslims resident in an EU country must decrease by at least 25% as a result of systematic and hostile efforts by the government or some other group.
Any country that has ever been a member of the EU (so the UK as well), or is admitted in the future, counts, even it later leaves or the EU is dissolved.
Question inspired by https://en.wikipedia.org/wiki/Expulsion_of_the_Moriscos
@NicoDelon I mean, there was a genocide of Muslims in a now EU-candidate country 28 years ago. The current conditions appear somewhat stable across the continent, but human nature being what it is, anything can happen. The monkey just needs a grievance and an outgroup.
@lukres But that’s the thing. If such a country is then admitted they have huge incentives not to that which would most likely cause them to be expelled from the union. I’m not saying it can’t happen. Just skeptical of the pathways.