- 🇩🇪Germany ro-no-lo
For anybody comming here from Google, like me, you can achieve this with the jsonapi_extras module. There is config page "JSON:API Resource overrides" were you can choose to override settings. To do so you choose the entity and click the override button. Then choose the json_raw field of your liking and go to "Advanced" and pick the enhancer "JSON Field".
After that, at least for me, the result of an json_field in the JSON:API was pure JSON.
(I still have to test, if the POST and PATCH will handle the pure JSON as well, but ready pure JSON is a great start)