This sounds like it may be a people problem rather than a technology problem. If a problem is moving on without him on a messaging platform I don't know why his teammates wouldn't do the same on an email thread or a github issue. One-liners also don't have to be the norm and I myself have gotten plenty of page long responses when warranted. Channels and threads also work for me when it comes to skimming, and Slack has a solid search feature which manages to make locating things super easy