Akin I like your line of thinking, but don’t think it is sensible for bunq to add this.
First of, payments should be instant and bunq is already working hard to make this reality (bunq support for instant payments already).
Second; with every form of proof I can think of right now there is a huge fraud risk (a page can be easily forged to trick someone in thinking that there is payed).
It is really hard to provide real proof of payment until you see the money in your account.
Why would someone believe a webpage of any bank/financial institution they may of may not know?
This is exactly why blockchain technology was invented. Although in my opinion this is not a valid solution as you need some kind of cryptocurrency and is not (yet) available for regular fiat money (and using cryptocurrency would mean someone needs to accept the currency risk).