Why were applets deprecated in JDK 9?

Applets were very popular a couple of years ago, but now the browser world changed and security is becoming a major focus for all major browser vendors.

The Java team gave its complete set or reasons, alternatives etc. in the document Migrating from Java Applets to plugin-free Java technologies. On page 4, there is the following Executive Overview:

With modern browser vendors working to restrict or reduce the support of plugins like Flash, Silverlight and Java in their products, developers of applications that rely on the Java browser plugin need to consider alternative options. Java developers currently relying on browser plugins should consider migrating from Java Applets to the plugin-free Java Web Start technology.

Supporting Java in browsers is only possible for as long as browser vendors are committed to supporting standards based plugins. By late 2015, many browser vendors had either removed or announced timelines for the removal of standards based plugin support, while some are introducing proprietary browser-specific extension APIs. Consequently, Oracle is planning to deprecate the Java browser plugin in JDK 9.

The deprecated plugin technology will be completely removed from the Oracle Java Development Kit (JDK) and Java Runtime Environment (JRE) in a future Java release TBD. Java Web Start applications do not rely on a browser plugin and will not be affected by these changes.

In short, this is because many main stream browsers had either removed or planned to remove standards based plugin support, therefore Oracle wants Java developers to migrate from Java applets to plugin-free Java Web-start technologies.

Oracle published a paper called "Migrating from Java Applets to plugin-free Java technologies " it explains in detail of why Applets are being deprecated and alternative ways developer should use.

I see Suresh quoting the executive overview below so I won't quote again.