Why NpgsqlEntityTypeBuilderExtensions and NpgsqlPropertyBuilderExtensions has ForNpgsqlHasComment method to set Comment, but NpgsqlIndexBuilderExtensions doesn't. How to set Comment for Index in FluentApi?
This isn't currently supported. You can open an issue requesting this on https://github.com/aspnet/EntityFrameworkCore/ (comment support has no been added to EF Core itself and will no longer be Npgsql-specific).
In the meantime you can use raw SQL in migrations to add comments to your indexes manually.