Java 15 is yet in very early development phase. Java 15 is expected to be released in September 2020. Till date we have following confirmed features in JDK 15.
Hidden Classes - hidden classes will be non-discoverable classes that will be generated and used dynamically when required through reflection.
ZGC - Z Garbage collector will now be a permanent feature which was an experimental feature in earlier releases.
Shenandoah GC will also be a product feature in JDK 15
Text Blocks as permanent feature in JDK 15