In Peer review 2.0 thread, from 2021, Martin Sleziak suggested to me this ten years old question. So, there is Papers$^\gamma$ with source code under CC0 Public Domain Dedication. It gains the popularity slowly, in an answer to another MO question I explain possible reasons behind this and give more examples of online platforms devoted to paper discussions.
Disclaimer: I'm the founder of http://papers-gamma.link.