We always prevented to add such checkbox. We wanted to avoid to loose users if they do not find a supplier because of a not checked box.
But we have more and more features that we add for supplier only on the party like Issue 9420: Add default lead/delivery time for product and suppliers - Tryton issue tracker. And I think having a supplier tab on all the parties may also loose the users.
So I think we could add such checkbox that will display or not the supplier tab on the party but without enforcing it with a domain on the purchase party. Instead we could have a warning/error when such purchase order is quoted to warn the user that it is not an “official” supplier and so it may not be correctly configured.
Also we should neither put a domain on the product supplier for the same reason.
What do you think?