The alternative in most cases is _no documentation at all_. It's not like people would go spend two hours writing up documentation if Slack didn't exist.
In large teams it's worth the reduction in 'code velocity' to have engineers document what they are building.
I specifically make efforts, and encourage others to do the same, to keep most documentation outside of slack. Do you have a question about a ticket? Ask it in a comment not slack. Do you have a question about how our system works? Ask it in the private Stack Overflow. Want to advertise a cool new internal service we can use? Great tell everyone in slack, but also add it to swaggerhub.
I specifically make efforts, and encourage others to do the same, to keep most documentation outside of slack. Do you have a question about a ticket? Ask it in a comment not slack. Do you have a question about how our system works? Ask it in the private Stack Overflow. Want to advertise a cool new internal service we can use? Great tell everyone in slack, but also add it to swaggerhub.