Exactly like email or the web. Certain services (shame on you, booking.com) now consider it a potential bug if you email doesn't end in gmail.com (they do their "did you mean: tadzik_@gmail.com?" when I enter the proper one, with my own domain). The web itself also tends to gravitate towards either "cloud" providers or at least stuff like cloudfare for ddos protection. Yes, it is still decentralized, but people keep choosing the centralized subsets of it for convenience.
This is incredibly annoying, I agree, but having run a service for which people type their email addresses, and having seen just how many people can't actually type their own email address properly (currently running at about 5%) I'm not surprised that a service aimed at non-technical people puts in attempted safeguards like this.
And my service is for technical people - I can't imagine what it's like for genuinely open services.
Omitting letters in their username, getting the domain wrong by omitting or mutating letters, commas instead of periods, spaces, and more. Some, such as commas and spaces are easy to catch, but mutations in the username are pretty much impossible to catch.
Without digging through my records I can't be more specific, but in general I was horrified at how many can't actually spell their own names correctly. Seriously, one person typed jojn.dpe instead of john.doe (name deliberately changed to protect the user). Sometimes it's possible to guess the correct username/domain, but sometimes it simply isn't.
It's funny because those are perfect examples of what they are talking about.
Gmail and Cloudflare.
For example, the web makes it cheap and trivial to DoS someone. And you often don't want your origin exposed. Hence everyone using Cloudflare and the collapse of the decentralized dream.