It's arguably the case that Firefox Relay is no different from the other services in this list. If someone can use it to create ~unlimited identities for almost free, that serves the same purpose as any other disposable email service.
And frankly, there's no sense in getting upset about a directory of services that allow the creation of unlimited disposable identities. If it wasn't this github repo, it would be another. As long as there's demand for lists of these services, the lists will exist.
And why do folks want lists of these services? Because they're a massive nuisance: folks avoid paying for services and abuse free services with spam (or worse). This isn't to say that businesses shouldn't support their users in being privacy-conscious, but the unfortunate fact of the matter is that Sybil-esque attacks are costly to businesses (time, resources, reputation), sometimes to the point of being existential threats. Many businesses have no choice but to make _some_ tradeoff between user-friendliness and revenue/operations cost.
My own business was absolutely _flooded_ with spam until I started blocking most popular disposable email services. I built tools to help detect and mitigate the spam, but it was purely reactive. I was spending more than half of my time fighting spam instead of working on the product.
The reason these services work for privacy-conscious folks is because they're convenient. But that's the same reason they're useful for malicious actors: nobody wants to spend the time to set up a new gmail account. The (time) cost of establishing the identity is what makes it hard to use for abuse.
There are obvious potential solutions. The first is for services like Firefox Relay to allow abuse reports. If I see spam from a Firefox Relay user, I should be able to report it and have it count against them. Any legitimate service should hold its customers to some responsible use guidelines, and if they break those rules, there should be consequences.
Another is that identities should not be "free". I disagree with Mozilla's pricing: $1 for unlimited identities is practically begging for bad actors to use the service. Setting limits or scaling the cost of the service (more identities per unit of time costs more, or throttling identity creation) makes it harder to abuse services with those identities. Almost no legitimate users actually need truly unlimited email addresses, and the ones who do can very much afford to pay more than a single dollar. Even generous limits would help to avoid many problems.
I'm generally bearish about crypto and blockchain technology, but I can also see how using it for identity would be super beneficial. There's some cost to establishing the identity, and you separately can use a (~worthless) disposable email for communication. The identity is worthless to "leak" and isn't useful for marketers.