LegacyDocument was never there to handle the “anything” case. It handles documents produced by the Java 1st gen SDK, that’s it.
Before flag uniformization in the second generation of SDKs (of which Java 2.x is part), each SDK would more or less do its own thing, transcoding wise. You couldn’t share a document between languages, and the effort went into making new generation cross-compatible within the same generation, not backward compatible with all other older SDKs.