To add to what Azie said, I would like to bring up another feature request from the past (Couldn't find the link, sorry): It was suggested to have a moderator's profile display when they obtained their rank. Now this itself is not related to your suggestion but one of the drawbacks mentioned was that certain members would look down upon newer moderators (Which has been confirmed by at least one moderator). And I believe this is also a problem with your suggestion.

New moderators already have it hard enough trying to learn everything they need to know, I don't think they need the extra nagging from members they kick/ban/whatever because they are ranked as a "Trainee".

I don't think this is super necessary. We already help new moderators in the way that you're suggesting; we just don't have a whole separate rank for it.

If your suggestion was meant to prevent small mistakes sometimes made more often by new moderators, I don't think an additional rank would really help with that. Different people learn differently and many people learn by doing. Mistakes that get made when someone is new are almost always benign and new mods are usually super apologetic and ready to learn when it happens. These mistakes can be fixed extremely quickly and are often resolved simply my messaging those involved with a short appology for the inconvenience.

These kinds of mistakes aren't just made by new mods either; I've been here more than 3 years and still make them occasionally.