If you want to migrate a plugin to GeoMedia Smart Client 2018 Update 4 or later, please follow the migration steps of 2018 Update 3 first. For more information see Migration Steps for GMSC 2018 Update 3.
GeoMedia Smart Client 2018 Update 4 or later updates the used OpenJDK and OpenJFX version to 12 or later. The detailed version info of the used OpenJDK is:
openjdk version "12" 2019-03-19 OpenJDK Runtime Environment (build 12+33) OpenJDK 64-Bit Server VM (build 12+33, mixed mode, sharing)
Therefore please ensure that your plugin doesn't use any Java API which is was marked as deprecated for removal of Java 11.
You will find a list of all items at the following page: