LSP Plugin Writers Guide

So a plugin sounds like something to can add to LSP. So the LSP Plugin Writer's Guide explains that?

Basically, yes. A plugin here means an "Analysis Plugin".

This has been mentioned before, in the VDMJ HLD, I think?

Well, that's part of it yes. The idea is that an "analysis" is a separate phase of processing that a VDM specification can be subject to. Some analyses are built-in, like the syntax and type checking; some are written externally and added as plugins.

But didn't this have something to do with the class mapping idea in the HLD?

That is true for the main analyses, like syntax, type checking, the interpreter and the PO generator. Those things have specialized AST classes that are mapped from somewhere else (ultimately, from the parser).