I've noticed that in case the check `java --fulversion` fails, hardcoded installations of JDK8 are done in the launchers:
https://github.com/jenkinsci/ec2-plugin/blob/da65b27d65164b3d8f43ce45f99ed73e5d85009b/src/main/java/hudson/plugins/ec2/ssh/EC2UnixLauncher.java#L238
https://github.com/jenkinsci/ec2-plugin/blob/da65b27d65164b3d8f43ce45f99ed73e5d85009b/src/main/java/hudson/plugins/ec2/ssh/EC2MacLauncher.java#L224
(didn't found the check for the Windows launcher)

As JDK8 support will be dropped in the next LTS (already dropped on weekly), these installations should be to be changed to JDK11.

Another solution would be to replace these hardcoded installations in case java isn't installed by an error message indicating java must be installed on the agent in order to use the plugin. (Avoid forcing a JDK > 8)

Note: this will be certainly a breaking change.