On: I patched my Slack client to keep “oops” messages ...

[strken]

Yeah, but there's also defence in depth, right? Being unable to delete a message means you're exposed to anyone who can see your main Slack channel for the stretch of time between posting and rotating, as opposed to the smaller number of people who saw the message or who are running a weird patched Slack client.

[trasz → strken]

This isn’t defence in depth, this is security by obscurity.

Also, honestly, if you are manually typing passwords and copy/pasting private keys, you can’t expect any security anyway.