I have 3 playlists synced to my iPhone: The two regular playlists (as matches for the rules in this photo), as well as the smart playlist created by the rules in the photo. In other words, the smart playlist shown is a subset of other music that is synced to my iPhone.
Will the smart playlist with live updating enabled automatically update itself to contain 8 hours of music selected by least recently played without syncing to iTunes?
Update: Seems to be a definite "Maybe".
I ran the playlist defined above 2 days in a row and got multiple repeats on the 2nd day of songs that were played on the 1st day.
The playlist is limited to 8hrs by least recently played of over 24hrs of available music. The playtime each day was less than 90 minutes.
This is what the playlist looked like on iOS 13's Music app. Perhaps the "Updated Yesterday" detail has something to do with it.