Java Applet and WebStart functionality, including the Applet API, The Java plug-in, the Java Applet Viewer, JNLP and Java Web Start including the javaws tool are all deprecated in JDK 9 and will be removed in a future release.
The actual reasoning behind the move is not public, and there is no indication as to which future release WebStart will be removed.
There is a related discussion on the OpenJDK discuss list here: http://mail.openjdk.java.net/pipermail/discuss/2017-November/004586.html
-Tim