Applets (as applications written in Java and intended to run inside a browser) are not deprecated in any way in HTML specifications or drafts. There is decreasing interest in them, due to many other alternatives being available, but that’s a different issue.
applet element was declared deprecated in HTML 4, in favor of the
object element. In HTML 4, deprecation means that element is still part of the language, browsers are recommended to support it (though in practice, not all browsers support it, and some browsers could not support it), but there is a recommendation to use something else (in this case,
object) instead of it.
In HTML5 CR, the word “deprecated” is not used. It uses the term “obsolete”, which means in principle something quite different but comes very close in practice. In HTML5 CR, the
applet element is declared “entirely obsolete and non-conforming”, with the note that it “must not be used by authors”. Yet, HTML5 contains a definition of
applet under “Requirements for implementations”. And HTML5 conformance requirements specify that normal browsers must (not just should) support it.
One of the few real differences between the HTML 4 concept “deprecated” and the HTML5 concept “obsolete” is in validation: when validating against an HTML 4 DTD,
applet is accepted when the Transitional DTD is used (but flagged as an error when validating against the Strict DTD); in HTML5 validation,
applet is reported as an error.
P.S. W3schools should not be used as any kind of authority or reference, see http://w3fools.com