Those interested in that, may love this: https://fermatslibrary.com
SciRate seems to be trying to be that. It's not buzzing yet, but the UX has its heart in the right place. Unfortunately, I see from other comments that the site is pretty old and never quite managed to take off.
I wonder if my dream is ever going to happen... sigh
What would make _you_ contribute to make SciRate more buzzing?
And even back when I was a grad student, I wouldn't have been willing to bear all the weight of making all these discussions happen by myself. There have to be other people active on the platform to make it worth using.
There's a big chicken-and-egg problem, I suppose. But some subreddits, like r/MachineLearning, have gotten us off the ground at least.