Is there a migration from the “Focal Point Field for Craft“ plugin to the Craft3 native focal point? Or has anyone tried this themselves?
The inherent problem that you're going to have is that Craft CMS 3 focal points are per asset, and Focal Point Field is per field.
So you could have multiple Focal Point Fields that have different Focal Points for the same Asset... and then how would you map them?
A migration could be done, but there would have to be some compromise to take the above into account.