How can I find somebody to chat about papers? I’m a computer-science PhD student. I want to find people to chat about papers (by others) online. How can I go about this?

I was looking into the chat of this website. I was also looking at some IRC channels.
Ask your supervisor and other people at your institute if something like this already exists, maybe you can join an existing group.

If it doesn't exist yet, maybe you can get their endorsement and assistance to set one up. It could help if they spread the word a bit even if they don't have time to join themselves.

Another avenue to explore is Meetup, trying out a variety of keywords in your field. Although that might end up being a bit more industry. Which doesn't have to be bad, if you had thoughts about going into industry after your PhD.