Arch package is configured with ICU support, but I think you need to use ICU collations explicitly: https://postgresql.verite.pro/blog/2018/07/25/icu-extension....

  SELECT * FROM pg_catalog.pg_collation;

As usual with collations, you must be careful and know what you are doing (old collations are also still available, for backwards compatibility)

Postgresql 10+ has added icu as a collation provider. If you select icu as your provider this is not an issue. If you select libc as your provider than this is still an issue you need to account for.