The comment here around {{outlet 'foo'}} being dropped as API was discussed on July 23rd with Ember Framework Core. We agreed that {{outlet 'foo'}} should become an assertion in dev builds. In a production build, 'foo' is an argument passed where {{outlet}} would expect none, and so it would be silently ignored. Asserting in dev will prevent users who expect an argument here from stumbling into an unexpected behavior.

edit: Implemented at #19691