For Java 11 or earlier execution environments, nothing is changed at the moment: an heuristic list of system packages is still listed statically and used, this lists weren't changed.
For later Java versions, the list of system packages are the ones exported by your known JVM installation that best matches the EE, the exact same packages that are visible to OSGi at runtime. The command below (that you can reproduce locally), lists for example the system packages I have locally with OpenJDK 14 shipped by Fedora:
$mistria@localhost:~/git/org.eclipse.tycho$ java -version && java -jar tycho-core/src/main/resources/system-packages-companion.jar | sort
openjdk version "14.0.1" 2020-04-14
OpenJDK Runtime Environment 20.3 (build 14.0.1+7)
OpenJDK 64-Bit Server VM 20.3 (build 14.0.1+7, mixed mode, sharing)
com.sun.jarsigner
com.sun.java.accessibility.util
com.sun.jdi
com.sun.jdi.connect
com.sun.jdi.connect.spi
com.sun.jdi.event
com.sun.jdi.request
com.sun.management
com.sun.net.httpserver
com.sun.net.httpserver.spi
com.sun.nio.file
com.sun.nio.sctp
com.sun.security.auth
com.sun.security.auth.callback
com.sun.security.auth.login
com.sun.security.auth.module
com.sun.security.jgss
com.sun.source.doctree
com.sun.source.tree
com.sun.source.util
com.sun.tools.attach
com.sun.tools.attach.spi
com.sun.tools.javac
com.sun.tools.jconsole
java.applet
java.awt
java.awt.color
java.awt.datatransfer
java.awt.desktop
java.awt.dnd
java.awt.event
java.awt.font
java.awt.geom
java.awt.imjava.awt.image
java.awt.image.renderable
java.awt.im.spi
java.awt.print
java.beans
java.beans.beancontext
java.iojava.lang
java.lang.annotation
java.lang.constant
java.lang.instrument
java.lang.invoke
java.lang.management
java.lang.module
java.lang.ref
java.lang.reflect
java.lang.runtime
java.math
java.netjava.net.http
java.net.spi
java.nio
java.nio.channels
java.nio.channels.spi
java.nio.charset
java.nio.charset.spi
java.nio.file
java.nio.file.attribute
java.nio.file.spi
java.rmi
java.rmi.activation
java.rmi.dgc
java.rmi.registry
java.rmi.server
java.security
java.security.cert
java.security.interfaces
java.security.spec
java.sql
java.text
java.text.spi
java.time
java.time.chrono
java.time.format
java.time.temporal
java.time.zone
java.util
java.util.concurrent
java.util.concurrent.atomic
java.util.concurrent.locks
java.util.function
java.util.jar
java.util.logging
java.util.prefs
java.util.regex
java.util.spi
java.util.stream
java.util.zip
javax.accessibility
javax.annotation.processing
javax.crypto
javax.crypto.interfaces
javax.crypto.spec
javax.imageio
javax.imageio.event
javax.imageio.metadata
javax.imageio.plugins.bmp
javax.imageio.plugins.jpeg
javax.imageio.plugins.tiff
javax.imageio.spi
javax.imageio.stream
javax.lang.model
javax.lang.model.element
javax.lang.model.type
javax.lang.model.util
javax.management
javax.management.loading
javax.management.modelmbean
javax.management.monitor
javax.management.openmbean
javax.management.relation
javax.management.remote
javax.management.remote.rmi
javax.management.timer
javax.naming
javax.naming.directory
javax.naming.event
javax.naming.ldap
javax.naming.ldap.spi
javax.naming.spi
javax.netjavax.net.ssl
javax.print
javax.print.attribute
javax.print.attribute.standard
javax.print.event
javax.rmi.ssl
javax.script
javax.security.auth
javax.security.auth.callback
javax.security.auth.kerberos
javax.security.auth.login
javax.security.auth.spi
javax.security.auth.x500
javax.security.cert
javax.security.sasl
javax.smartcardio
javax.sound.midi
javax.sound.midi.spi
javax.sound.sampled
javax.sound.sampled.spi
javax.sql
javax.sql.rowset
javax.sql.rowset.serial
javax.sql.rowset.spi
javax.swing
javax.swing.border
javax.swing.colorchooser
javax.swing.event
javax.swing.filechooser
javax.swing.plaf
javax.swing.plaf.basic
javax.swing.plaf.metal
javax.swing.plaf.multi
javax.swing.plaf.nimbus
javax.swing.plaf.synth
javax.swing.table
javax.swing.text
javax.swing.text.html
javax.swing.text.html.parser
javax.swing.text.rtf
javax.swing.tree
javax.swing.undo
javax.tools
javax.transaction.xa
javax.xml
javax.xml.catalog
javax.xml.crypto
javax.xml.crypto.dom
javax.xml.crypto.dsig
javax.xml.crypto.dsig.dom
javax.xml.crypto.dsig.keyinfo
javax.xml.crypto.dsig.spec
javax.xml.datatype
javax.xml.namespace
javax.xml.parsers
javax.xml.stream
javax.xml.stream.events
javax.xml.stream.util
javax.xml.transform
javax.xml.transform.dom
javax.xml.transform.sax
javax.xml.transform.stax
javax.xml.transform.stream
javax.xml.validation
javax.xml.xpath
jdk.dynalink
jdk.dynalink.beans
jdk.dynalink.linker
jdk.dynalink.linker.support
jdk.dynalink.support
jdk.javadoc.doclet
jdk.jfr
jdk.jfr.consumer
jdk.jshell
jdk.jshell.execution
jdk.jshell.spi
jdk.jshell.tool
jdk.management.jfr
jdk.nashorn.api.scripting
jdk.nashorn.api.tree
jdk.netjdk.nio
jdk.nio.mapmode
jdk.security.jarsigner
jdk.swing.interop
netscape._javascript_
org.ietf.jgss
org.w3c.dom
org.w3c.dom.bootstrap
org.w3c.dom.css
org.w3c.dom.events
org.w3c.dom.html
org.w3c.dom.lsorg.w3c.dom.ranges
org.w3c.dom.stylesheets
org.w3c.dom.traversal
org.w3c.dom.views
org.w3c.dom.xpath
org.xml.sax
org.xml.sax.ext
org.xml.sax.helpers
sun.misc
sun.reflect
Looks like some sun packages are still there; but again, now it all depends on your installation as JVM are modular and can differ from one installation to the other.
But also, I'd like to clarify that what Tycho does now is more consistent with what OSGi/Equinox does at runtime, so something that would fail now would most likely highlight a bug that would show up at runtime (while Tycho used to be silent about it).
