A well-known iOS developer, Luca Tudesco, is reported as stating that jailbreaking in the iPhone XS Max's A12 chip is possible. It may be a follow-up in which the related group Pangu claimed that they had done the same in iOS 12. However, there is no evidence that this option is more generally available yet.
https://www.notebookcheck.net/Developer-claims-to-have-jailbroken-iPhone-XS-Max.337624.0.html
With this good news I'm finally willing to take the risk of moving my jailbroken iPP 9.3 on to 12.0 and wait for this release.