10 years ago there were a dozen sites dedicated to flash games that raked in millions of views per month. I couldn't name you a single WebGL site dedicated to games.
Where is the modern day Newgrounds? There isn't one. It doesn't exist. And that's a damn shame.
I'd say Newgrounds is the modern day Newgrounds. I don't have Flash installed, I went to Newgrounds, and the first couple of games I clicked on worked without Flash while others still required it. If there's new or old content that still requires Flash, then it can be emulated. Here's an example of a large collection of games playable in an HTML5-based emulator: https://archive.org/details/softwarelibrary_msdos_games
As for other HTML5\WebGL focused sites, here's an example of a site making HTML5 games: https://ga.me/
And here are a couple of sites indexing WebGL games:
Where is the modern day Newgrounds? There isn't one. It doesn't exist. And that's a damn shame.