Run the test on OS X with JDK9 b120:
javax/swing/text/JTextComponent/5074573/bug5074573.java

it will fail with Aqua LaF: neither Alt+Backspace nor Alt+Delete would have no effect in JTextArea.
However, running a simplest application with JTextArea or playing with the same test as a main application after robot finished, we could say the functionality works. I don't see any bug in the test. That may be a Robot issue.

Even simpler illustration I'll attach here. Robot will type 'a' and then Alt+Backspace; a listener will report all keyPress events; however the typed 'a' will not be deleted. Once I type Alt+Backspace by hand again, it will.

Perhaps it has something common with JDK-8157173.

And, it worked in b111, b118: so I'm afraid there's a regression -- I'm just not sure how grave one. Definitely it would block certain testing.