Asking "How do I unsend an email" is just as unreasonable as asking "Hey, give me that gift I gave you back".
The problem is that, regardless of your intent, you've given something to someone and they own it now.
You can't undo that without involving the 3rd party (Or breaking the law and stealing it, digitally for the email).
And I want to be clear upfront - THIS IS A GOOD THING.
I don't want Amazon to be able to "unsend" my receipts. I don't want Google to be able to "unsend" my support chat log. I don't want my boss to be able to "unsend" his approval for my time off.
This system is designed explicitly to put the USER first. But it treats each side of the exchange as user, and values them equally - If they disagree, it's not the system's job to resolve that, they have to talk it out.
Slack and Messenger (and a lot of other modern chat) have an arbiter - They don't require users to agree because they own the content, not the users. In a company slack, I haven't given you anything when I message you. I've just pinned something to the company message board. I can take it down, and so can anyone else who has the key. It's not mine and it's not yours - It is very clearly owned by the company.
That can work great in a well structured environment, but I have to laugh when you say its replacing email. It's not a replacement, it's a complement - They are not the same things.
Just like an oral promise with no witnesses is NOT the same as a signed receipt. Use each as needed.