Here's a list of several hundred(!) games in the IO format: http://iogames.space/

This is the so called "io game" genre, started by agar.io.

The new era of Flash-type games?

"io games": Massively multi-player, real time, no login, no rounds (continuous game), just type your name and play.

There probably were a few flash games like this, but didn't caught on like it has now.

How do they pay for server cost? Someone told me slither.io had made it's owner millions. I'm assuming there are ads?

Yea they show ads in between game sessions.

Casual Collective with their robot battle games from 10 years ago. Though they did have more configuration, you could just add your name, pick a bot and play.

http://www.kongregate.com/games/CasualCollective/minions

http://everybodyedits.com was one of the first MMOs I played with no config (except a name and a map to join). In addition all of the maps were editable. So it was unique in that every map was completely user created.

What I said... It's not a new idea, but it didn't made a genre back then. It was a very different internet world, too.