Is there a rule to hide the new HUD at session start?

The "Show/Hide HUD Panel" and the "Display Custom HUD" session rules no longer seem to work in TRS2019. Is there a replacement ?

There aren't direct replacements for those. The TRS19 UI displays basically all the same information as the Custom HUD did, the main exception being the ability to show a variable. That can be done in TRS19 using the "Set Subtitle" rule.

The UI is hidden whilst children of "Activate cutscene mode" rule are running. The player can skip the cutscene at any time though which will end those child rules and restore the UI, so you wouldn't want anything necessary for gameplay nested under the "Activate cutscene mode" rule.

OK, thanks Rob.