My apologies if I’m missing something but when searching for polymorphic relationships in Ember I found quite a few blog posts and some forum posts discussing it but I can’t seem to find the official documentation? The API documentation seems to suggest that only async and inverse are valid options: Ember - 4.6 - Ember API Documentation
Has it been removed, is it considered unstable or something else?
To address your instability concern I see a number of open issues in the repo, so I’m guessing it may not be the most well trodden path. Ember data can be a bit rough around the edges, my motto has been don’t do anything remotely unusual unless you’re willing to spend time understanding ember-data internals to do custom overrides or fix bugs.