Its proprietary, but trivially decompileable. Sure, maybe if the game was released today they would try obfuscating better, but the thing is that modding culture had already basically torn the game apart by the time Microsoft came around. I don't think they had a choice but to accept the status quo.
There are mods to disable unwanted Microsoft-pushed features like chat signing, for example. Microsoft doesn't want this, but it is so trivial to do with the Java edition they don't really have a choice but to either accept it or legal it.
I am also surprised they didn't drop mac/linux support, but as long as maintaining it is free I don't think they will. They have lost a lot of market share to mac after all - that's a lot of customers to miss out on.
Ok I had to look this up because last time I did MC modding was when I was in high school... It used to be obfuscated. Someone used to manually map those to human-readable names (didn't know this back then and thought the names were somehow hidden in the bytecode). But recently MS has started deliberately shipping unobfuscated builds.
So I'm getting the feeling they actually want JE to be moddable. Which makes sense because they're already successfully pushing Bedrock on a majority who don't care about modding, especially on locked down platforms, and the remaining JE players are a different segment. But that could change just as easily as their leadership changes.
But also just realized, if this changed, rather than locking down JE, they'd probably just kill it and maaybe release Bedrock for Mac/Linux to keep those players. Possibly not.