I suspect monzo are focused on developing tools that empower the user to better manage their finances (as that’s good for PR). An overdraft pot sounds great on the surface, but launching a feature like that could be seen as encouraging spending of money you don’t have. I think other features like bill pots, salary sorting, virtual cards from pots, flex for larger purchases; all work to avoid being in a situation where going into an overdraft is necessary.
What spending are you worried will push you into your overdraft? If its a bill, those can already be set to come from a pot using bill pots. If its another type of small spend then you could use a virtual card with monzo plus and have that come out of a pot. If its a larger one-off purchase, then flex could be used to soften the blow.