A lot of people don't seem to be aware that the Stack Exchange network consists of lots of different sites. StackOverflow is just the biggest one, but there are sites for all sorts of discussion: https://stackexchange.com/sites

Those sites all work exactly the same as StackOverflow and you can login with a single account. Just the rules regarding content are different. See for example https://softwareengineering.stackexchange.com/

There is also the Area 51 site for proposals for new sites: https://area51.stackexchange.com/

Hacker News, Reddit, Quora, mailing lists, google hangouts, irc, slack or discord channels.

They're still around. They're smaller... and by being smaller they are more useful.

What Stack Overflow killed off was Experts Exchange and a smattering of forums. Some are still around, but they're not trying to be searchable. Reddit's /r/javahelp or Code Ranch - every time there's a question its posted as a new question. Few people go back to find old posts there, but then that's the way the community works... and it works because it is so much smaller than Stack Overflow.

Pretty much, or at least shrinked them a lot. If you make a initially better experience for 75% of the content, few communities manage to survive while only dealing with 25%. (ratio choosen at random, I have no idea what it looks like in practice)

IRC channels and Slacks have taken some of it, with all the downsides that comes with that.