This is a major drawback for changing to multiselection.
Well let’s agree we disagree.
Accounting department in a company is really a central point. Every invoice should go through it, we can not say the same for the others which may be split per warehouse, locations or business unit.
But indeed for sales and purchases, I think the most expected is that the placing party is notified (which may be different from the shipping and invoicing per Tryton design) and not a predefined address for all orders.
Also now we have a contact on those orders that we could allow to use for notification but for that we need to allow to use contact (limited to email).
No you do not need to use a custom module to use notification email for other document. They will be sent to the primary email address of the party, user or employee. This is a default behavior that sweet most of the cases.