Same camp here, even more if this is a startup, these once-optional keys and settings will be ossified into the database, the knowledge of why they existed will

Same camp here, even more if this is a startup, these once-optional keys and settings will be ossified into the database, the knowledge of why they existed will be lost (as most knowledge from startups after 5 years end up), untangling that mess will take an engineer's days to weeks doing discovery work of possible incantations of its usage, etc. If you give such a powerful feature to any table you are just shooting yourself on the foot on the whole modeling of your application, there is no static representation of the model, it will be dynamic depending on the JSONB configuration.