This critical feature was requested back in 2020 and it’s important enough to be worth whatever bureaucracy may be involved. 
So please consider our request diligently.
Comments/Notes are Everything when it comes to delivering a solid support experience. “Document everything” is the golden rule, but my team is deathly afraid of the comment feature after replying to a user/client instead of commenting on their case.
Here is one suggestion about how unsending can be added to the HubSpot inbox.
The mockup was done in MS Paint, so um, try to look past that.
The vertical ellipsis contains “Convert Reply to Comment” and could just as well contain “Send Now”, leaving only “Unsend” in the open. I imagine the countdown to be configurable, but if it gets the feature into our hands faster, we’d probably all be happy with about 5 minutes with the option to Send Now. It’s probably enough time to rush to someone’s office and tell them to unsend their message right away.
Thanks for your time. I hope that this will be implemented soon; our team needs it.