As far as I can tell with B4j V9.80, the layouts will still load, and if you have SceneBuilder installed, clicking on the layout in the File Manager will open it in the SceneBuilder app.
Obviously as this is deprecated, it will no longer be supported here, but it will give you a more time to migrate your layouts.
I do get a warning in the log :
WARNING: Loading FXML document with JavaFX API of version 18 by JavaFX runtime of version 11.0.1
Presumably because I have just downloaded the latest SceneBuilder to test it and am using Java 11 not Java 18, but it appears to work.