The new build has number 2.45.709.1 and requires a 1.06GB download. The changelog isn’t particularly detailed, only mentioning system enhancements and the removal of Google+ from BlinkFeed.
That said, this new firmware isn’t devoid of issues. Early adopters report it has a bug which prevents the users from editing the APN.
The Pie update for the U12+ is currently seeding in Taiwan, and it should roll out in other regions soon if everything goes smoothly.