It's primarily used for schema dumping, we're removing just about everywhere else that it matters. It also affects XML serialization, but I'm not sure we have defined behavior for custom Postgres types there. We could support it in schema dumping by adding a method here:

https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb#L80-L146

Though we should probably have the dumper generate `t.column :foo, :some_type` as a fallback in general