Depends. Our engineering slack (Few thousand members) doesn't contain secrets for a few reasons:

1. Secret scanning
2. We have a /secret bot that will take your secret, store it securely, and then present a GUI for each person with access to display that secret "for just that person". And then after a set period of time it's made inaccessible, and wiped from the infra.
3. Training and knowledge transfer on secret security

This has been incredibly effective. Especially the secret bot.

Turns out that the problem with people sharing secrets is just a matter of convenience. If you make a secure way convenient then everyone tends to just use it by default.