The stuff about etiquette in this is bullshit frankly, but it does raise some interesting questions. The primary duty-bearer for playing a decent wage to delivery staff ought to be the employer. It's clear that they're often failing in their duty. https://www.bbc.co.uk/news/business-54410391

However, I suspect that if the employer fails to carry out their duty of paying a decent wage, the customer benefitting from the service becomes a secondary duty-bearer. Thus, there's a duty to tip where wages are poor.

However, shifting the onus onto customers to ensure delivery drivers get paid a decent wage might normalise the practice of employers shirking responsibilities. Ideally, we need employers to pay a proper wage and the state to do its sodding job enforcing that.