This function is already there on Mac, iPad and iPhone Max (in landscape mode)
Note! In the iOS/iPad note list, there is a special “hidden” trix:
Tap and hold first note, then select more notes with a second finger, and finally drag this “note stack” over and drop it onto a tag in the left tag panel.
The tag to apply (drag to) has to to be there already of cource.
But I would love to be able to assign a NEW tag to multiple selected notes.
Please count my vote on that @trix180 and @matteo 