It seems like the slack part of the PR project has been retired without being removed from the project page. proposition: clean up the page and remove the dead links.
I've updated the page and removed all the references to slack. @gentoo.org slack joining still works but there are 5 people there, looks pretty dead to me. assigning to pr@ to handle the rest as they see fit.
edit history is here https://wiki.gentoo.org/index.php?title=Project:Public_Relations&action=history