First line in the above link says, that Java 8 is minimum and yet the other line says that it can work with Java 7 update 55 or later.
Yes. It's a bug.
In 2.x branches Java 7 is ok. Although Java 8 is recommended.
Wanna open an issue or send a pr ?
Sure... But can you show me how to do either one of those please? Not done it before.
Here is the issue: https://github.com/elastic/elasticsearch/issues/15261