I never bother reading slack backlogs. I use slack to get quick answers to questions and then for any kind of design decisions, they go in to a diagram or wiki page which will be discussed in the next meeting.
Quite, slack is the equivalent of an office conversation, you can join in or ignore it. If someone wants you specifically they can @ you, and you can briefly glance at the context over the last few lines. Usually at that point you'll jump into a huddle / call, maybe you'll have to go back over it, but it only takes a few seconds to get the gist of what's being talked about, far easier than getting the gist of an office meeting that you've just been pulled into.
I don't see much point in having more than a few kilobytes of text in the scrollback. If it's persistent, put it in jira.
