jabel VS Class-Version-Patcher

Java has things like Jabel that let you use modern Java syntax and compile down to Java 8 for compatibility reasons.

https://github.com/bsideup/jabel

This is useful commercially, but also nice when building Minecraft mods for older versions of Minecraft.

As for Java bytecode, you assume the differences are significant while in reality, they are not, as explained by the Jabel project:

(Before realizing it'd already been done)

If you want to use new language features (pattern matching, string templates, records etc.) then I highly recommend to take a look at https://github.com/bsideup/jabel

I've never heard of somebody sticking to 9 or 10 as their primary JDK. AFAIK most people who could update waited for the 11 LTS. The rest of us are just stuck on 8. But we have some nice tricks.

https://github.com/bsideup/jabel - lets you use new Java syntax, because things like pattern matching are just syntatic sugar and dont change the class file specification

I've found about the Jabel that tried to enable this feature in the Javac compiler.

I made one (Before realizing it'd already been done) to backport newer language features to Java 8 and it was pretty easy to bundle it up as a maven plugin. But going back all the way to 4 will be a notable increase in difficulty. Retrolambda goes to 1.5 but not all the way to 4. You could hypothetically add a layer on top of it to support 1.5 -> 1.4. That or roll a fork of it.