But for certain businesses, there is a case to be made for not serving these kinds of customers. Someone using a shitty old Android phone might not even be able/willing to pay for your product, and if they do, might cost more in technical support and/or fraud (due to their device being vulnerable) than what they bring in revenue.

When you are a bank or a crypto exchange, that might make sense. When you are the municipality of a village in rural India or the company that handles all vehicle registrations, it does not.

You cannot gatekeep with such sweeping statements. People have old phones for lots of reasons. Others will have to serve those people for lots of other reasons.