You're talking about the login flow. OP is talking about onboarding.
Go to discord.com right now, click "Open Discord in your browser", enter a random username, and hit enter.
That's a pretty recent change though. Not long ago they used to require a non-temporary email, often a phone number (no prepay phone numbers allowed, ruling out most of many countries), and maybe a bunch of captchas to top it off.
I actually remember this being a thing back in 2015-16 when they first launched and before the "boom" happened. Back then it was only possible when you actually got an invite from somebody to join a server though, if I remember correctly.
But back then there was no official way to browse more or less public servers, which is the case now. I guess to join those it might be good enough in their eyes to just create a temporary account to let users test the waters and hopefully later convert into a proper account.
It's not recent at all. That's how I first used Discord back in 2015.
That's weird, maybe it's disabled if they are already suspicious of your IP for some reason? I remember having no end of trouble trying to use it while travelling a while ago.
Are you sure you weren't just trying to join a server that required extra verification? My first experiences with Discord, years ago, were exactly the same as the new user experience now, except they seem to be running out of unique digits for some common names.
This is the Web experience at least, on mobile they presumably already have all of this (or the equivalents) right after installing their app.
EDIT: to add to this, I once created a Discord account associated with a fake e-mail. After a while they presumably figured that out, but instead of, e.g., letting me log in and change the e-mail, they just locked me out.