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Wide usage of internal only APIs is why much of the world is still stuck using Java 8 and can't migrate.

You mean “public APIs not being comprehensive enough”

I use java about 40% of the time at my work and I've never had to use "internal only" APIs. Do you have some examples of these internal APIs?

There are JDK Enhancement Proposals that discuss the encapsulation of these internal APIs, including some examples and alternatives:

The sun.* package hierarchy for example: