"Enable delay send - Allows you to undo sending emails for 5 minutes", I'd argue that a lot of people would enable that pretty fast.

Keyboard shortcuts probably would work like I'd expect, people like me would go "Hell no, no keyboard shortcuts in browser application EVER", and power users would opt into that in an instant.

Allow you to undo sending for 5 minutes means email delivery is delayed by 5 minutes.

It doesn't have to be 5 minutes. It could be 15 seconds. I've used that feature (in Fastmail), and find it very valuable.

I'm confused. Doesn't gmail offer an 'undo' for send by default ? At least for the last 5 years ? It's in General settings "Undo Send" and can be set up to 30 seconds ?

And before that it was under the Labs experimental features, I think I enabled it in the late 2000s.