Of course, if it's designed to be extensible then definitely non-public sources could be added, but whether they'd be supplied as included modules/plugins or ne

Of course, if it's designed to be extensible then definitely non-public sources could be added, but whether they'd be supplied as included modules/plugins or need to be developed (and then optionally contributed upstream) by users of a given non-public service is a question of practicality and sometimes cost (if it's a paid service an OSS project may not be able to justify the cost of a licence/account for the service to develop/test against).