If you must tip, only ever use cash. Otherwise you're also subsidizing Square, Toast, Visa, Stripe, etc. Did they give you exceptional service today? Use cash.
Unfortunately, culture in many countries is to tip in certain situations, especially in restaurants, even for normal service. But just because it's culturally normal to tip in restaurants in your country doesn't mean you'd tip a giant corporation like Visa. That's not how culture works.
Crypto claims to be a solution to this, but in practice approximately all crypto transactions go through a middleman (exchange) which takes a cut. It's unreasonable to expect your barista to go through the effort of accepting crypto payments without involving an exchange.
Cash doesn't have this problem.
I'd love to be proven wrong about this. Is there a digital payment method that runs on phones that doesn't subsidize a middleman?