95% might have little value or zero but 5% of them are gold, it’s just not always clear which 5% is the gold until you need it.

Slack is the first place I search for any issue at my company and I frequently take advantage of 3-4 year old threads

In Hack Club, a lot. I'm a teen in HC, many projects run for months and have very valuable messages for a long time.

Adding on to this, a lot of people don't want their personal chats deleted either!

I'm actually part of some Slack workspaces that are on the free plan which hides messages (including DMs) older than 90 days. It is actually quite cumbersome then because if someone sends a valuable message, I have to remember to screenshot or better yet copy-paste it into a durable spot or else I'm going to have to ask again about the same thing.

In BC, engineering firms are legally required to maintain project documentation for 10 years, including slack messages.

We use Slack extensively and I'm searching for info in conversations from months or even years ago regularly.

Slacks biggest value is ephemeral nature. Forces you to document in proper places.