The Importance of Partially Deploying OTP

I haven't really had anything of a break necessary for E-mail. We had one time where we updated an OTP version and that had an odd interaction with Hackney, I think, like something broke. So we actually were forced to revert back to an older OTP version. That was also was only caught in our cart and production. And at that point, I'm also not sure what to do best. But if I could choose, I would love to have something like that where you can say, hey, I'm already a deploy the new version of this. But I want to keep it running alongside the old one for a while,. Maybe to see if it's not