The way I have dealt with this issue for the past decade+ is to simply offer multiple tiered pricing for my clients.
By default, the lowest price is design that supports the most modern browsers. The older and more problematic the browser support required, the more it adds to the project cost.
This cuts out 99% of all the old browser support issues in my business. Very rarely does someone need more than "just ok" support for even IE 11 these days. And that isn't so bad even with css grid/flexbox.
Supporting old browsers isn't a technical problem, it's a time/cost problem.
Yeah, it just depends on your audience. It's still a problem in B2B situations. Most notably when a customer's IT is contracted. Unfortunately for me it's not a matter of price tiering, it's about producing a product that has been working for customers in outdated browsers that continue to expect it to work that have signed annual+ contracts.
Negotiations and communication can help resolve these kinds of problems. (in my experience)
The core motivator to start the discussion can be simply profit margin. I've hit points where it's not profitable, and therefore gets cut from our offerings. But if someone wants to pay extra, we can accommodate.
Is it possible that by changing the requirements/costs of the product you would lose the account/business?
The way I started this discussion is with the phrase "web standards", and these are cheaper to build towards. So "non-standard" systems cost extra to support now. But this also meant I could lower my costs on some other work, so it wasn't smoke and mirrors, it was legitimately easier and less time consuming. (also, makes everyone happier to work on standards instead of spending most of our time with hacks and debugging because reasons)
By default, the lowest price is design that supports the most modern browsers. The older and more problematic the browser support required, the more it adds to the project cost.
This cuts out 99% of all the old browser support issues in my business. Very rarely does someone need more than "just ok" support for even IE 11 these days. And that isn't so bad even with css grid/flexbox.
Supporting old browsers isn't a technical problem, it's a time/cost problem.