JDK 9+ probing is incorrect when toolchains are used.

[pdudits]

When toolchains are used in the build, then java version the plugin is running in and java version it would actually execute may differ.

However, the probe for JDK 9+ is done in-process:

jaxws-maven-plugin/src/main/java/org/codehaus/mojo/jaxws/AbstractJaxwsMojo.java

Lines 509 to 519 in eeca039

	private boolean isModular() {
	try
	{
	Class<?> moduleClass = Class.forName( "java.lang.Module" );
	return moduleClass != null;
	}
	catch ( ClassNotFoundException e )
	{
	return false;
	}
	}

Therefore when maven runs JDK 8 and toolchains JDK is JDK 11, plugin will fail with

-Xbootclasspath/p is no longer a supported option.

Because decision to include the argument was made inside JDK 8 VM and then JDK 11 was launched.