There's the possibility to do it, but it is experimental and it won't be included in V4 as sub-domains will be used for languages as it showed better results. You could give it a try with usernames, but don't get used to it.
He answered that @jaakko . It exists. You just need to write the htaccess rules to enable it along with flipping the setting in the link Rodolfo linked.
It's pretty nice, but it does have a lot of issues. I find it breaks often so I stopped using it. But if you wanna write the htaccess rules go for it.