Advertising-supported media directly leads to native advertising, click-bait headlines and plagiarism, all of which decrease the quality of service.
I don't know what native advertising is, but click-bait headlines and plagiarism... that sounds more like a decrease in the quality of the adverts themselves. But as I don't click on those things anyway, I don't care about that. It's the quality of the actual email/site/whatever that I care about, and if I don't like that then I won't use it and won't see the ads, so it's in their interest to make the actual service good.
Do you think non-paying users should have a right to complain about anything?
Yes, for the reasons I just gave. They're not paying themselves, but their mere presence on the site generates revenue, so they are responsible for the income. It's not really about a "right", it's more that you can't stop people complaining about things they don't like, and it's in the company's best interests to listen to the complaints (or at the very least know they exist) so that they can keep people going there and keep generating ad revenue.
How about complaining that free service tiers don't give all the features of paid tiers from the same service provider? (i.e. complaining that free Spotify can't do everything that paid Spotify does)
I don't know if this question was addressed at me specifically, but that's not the sort of situation I was talking about at all so I have no comment here.
I think it's clear that some complaints about free services are frivolous, and I think that complaining about a phone number requirement, which has essentially no user downsides, while making it cheaper to provide a free service, is one of those complaints.
Doesn't really matter what you think about it. If enough people don't like it and boycott a service because of it, no matter how frivolous you consider that to be, it will have a negative impact on revenue that could have been avoided by paying more attention to what people want. And I don't really buy this argument that it makes anything cheaper. I don't see what difference it makes if an entirely automated services asks me what the name of my first pet was, or sends me an activation link to a different email address, or sends me a text to my mobile phone. No human was involved in any part of any of those processes, no massive labour was enacted, and they're all essentially the same thing at heart. If anything, you would think that a text message or phone call would incur MORE cost than either of the other methods as there's a mobile phone network involved in the process who will undoubtedly want some form of remuneration for the privilege.