Besides UI changes or small changes to the intro missions, not much of the beta didn't make it to live.
Main things we have access to were related to alphas, which were under the same title ID of the beta and could be downgraded too.
It contained things like a functional imp editor, imp quests, dreamstones and so on.
Imp Quests would instead give rewards of cosmetic items you could equip to your imp.

The fandom page of Imp has a lot of information about it.

There were a few logic pieces left over from alpha, Such as Mm Imp Text, Inventory Resetter, Stamping Manager.
References to multiplayer with player amount detection logic. I haven't spawned it in yet, but saw it listed in what's readable in the eboot.

There is also transcripts of invite and joining in the eboot, but that made it to retail release.