The temptation and the value is too great, and the opt-in opt-out consent thing ends up being a fuckery where the company tries to trick the user into allowing them to take a look into the data, presumably because they are selling the product at a loss and need an alternative revenue model.
Just make it impossible from the get-go, the fine print would be that the data can be shared off-band explicitly, in an email, or if explicitly copy pasted in a support chatbox, but there would be no mechanism for us to read the data from the databases much less from the client.
I don't mean it would be an air-tight mechanism like Signal or ProtonMail, if a court order would ask us to produce client info, we would still reserve the right to produce the data, but exceptionally, and definitely not for training models.