Basically, I feel we’ve all been scorned before having a cub we wanted age up and forgetting to give them a brood mother. I just propose a warning on the cubs page. Something like, this cub may die tomorrow if it isn’t protected.

And/or maybe the ability to choose a brood mother with available slots from the cubs page would help.

Edit to respond to possible counter arguments:
While there is the skull icon next to at risk cubs within the cave page, I believe it is easy to overlook, especially if the cub you want is among cubs you dont care about. A text box warning would be more helpful to those that need it, as you are more likely to click on the individual cub's page. it is also easy to ignore if you don't care for the cub.

There is already a very similar feature for un-nested pregnant lions, it would be no different.
The reason for this is that it would help players mitigate mistakes happening because of a bad memory or not being completely on top of which cubs are being kept.

And to clarify this: The ideal notification would be kept exclusively to the individual cubs page to avoid further congested den page notifs. A den page notif may also be annoying for cubs you don’t care about, so I reiterate that I would want this only for the individual.

While I agree there is some personal responsibility to be had, I think it is a fair balance, as it is still entirely possible for the cub to die. Cub death is very permanent and can be devastating. This is a simple reminder. It wouldn't negatively affect anyone, and only reaps positive benefits for those it helps.

I am not asking for this to solve cub death, as I believe it is a good balance to a relatively easy game. It’s entirely possible for a cub to die outright before getting an injured cub notification on rollover. I would be content even without a roll notif, just the banner would be enough of a reminder. I think almost every other type of danger that affects your lions gets a text box warning except for this.