ICU collations are not used by default, you have to explicitly enable them, and you can't set them as the database default. And even if you use ICU collations, I'm not sure if indices actually detect if the ICU versions don't match.
Maybe they improved some of the issues in PostgreSQL 11 (haven't been following the ICU issue in detail), but I don't think this issue is fixed.
ICU collations are not used by default, you have to explicitly enable them, and you can't set them as the database default. And even if you use ICU collations, I'm not sure if indices actually detect if the ICU versions don't match.
Maybe they improved some of the issues in PostgreSQL 11 (haven't been following the ICU issue in detail), but I don't think this issue is fixed.