Slack's "pay to search archives" feature is really ingenious. What content will people pay for? Content that they created!
I work with a lot of different companies. They almost universally have Slack instances that get used for discussion. One time I was working with a client who was too tight to actually pay for their Slack instance so it had amnesia.
For a while I was really grouchy about it. Then one day I was thinking along the lines of "Ah this is such a PITA. I've just had this good discussion about these requirements in Slack but because it's not a paid instance it'll vanish in a few days. Grr I'll have to take this information and put it somewhere else. I guess I'll stick it in a design document in the project's wiki. This is such an inconven... wait. Huh. That's vastly better than leaving it floating somewhere in Slack. Oh."
And in that moment, I was enlightened. Paying for Slack actually makes it a worse tool.
Like if it was just a big list of "Stuff people have starred in Slack" I would not use it.
If it was a repository of information categorised by project and subject, maybe with a little blurb of info to give me a summary of it, then under all that a link to the source material if I still needed it, that might be compelling.
So at least, for me, Skype for business has this functionality.
Personal skype stores your chat history local to the device- I found that out recently when I was looking at my non-existent conversation history on my new phone.