I think you'd be surprised. Especially for restaurants - there are mom-and-pop operations with smart kids who want to put together a project to make their parents' lives easier. Money isn't the only motivator in life and I'm not convinced you couldn't find someone to donate the capex needed to get an MVP off the ground. https://github.com/topics/food-ordering-system has 537 repositories, I don't have the time deep dive into them and whether any of them are any good, but I think it's evidence enough that FOSS can provide an MVP at least.

Yes, that's a different question from whether the opex is sustainable without a business selling support. And I think the question there is how much can be offloaded to utility-style SaaS like Stripe (i.e. handle changes in taxes) and serverless infrastructure to keep the opex of the FOSS solution itself low. I get the impression that it's hypothetically doable, but the proverbial kids of mom-and-pop store parents don't have the experience or guidance to know how to set that up.