At least in Brazil, it was very rare. In the last 3-4 years, it's almost every time you pay. And you have to grab and hold the payment terminal (especially if you're using tap / contactless payment) so the cashier or waiter, trying to be helpful, doesn't click the wrong button and cost you 15%.
Some places they're insistent that you must do currency conversion or the payment won't work. Makes me think the merchant must be getting a chunk of that profit and telling their staff to accept the conversion...
Plus, the point is that you're asked whether you'd like to pay more for something, where there is no benefit in it for you nor a public benefit etc.