Yes, unfortunately it seems that the Scintilla codebase is far from async-ready which will make it a challenge to support new stuff like LSP. We might get tree-sitter parsing support, though.
It's more or less a way for standaline plugins/programs to provide language-specific functionality like autocompletions, syntax-highlighting, and diagnostic errors (like type errors) independently of a specific editor.
The idea is that standalone programs are made called language servers which can provide these features. Editors tend to communicate with them using JSON-RPC over stdin. So one can only write a language-specific parser once and integrate it with any editor.
Usually (at least in VS Code), there's a third part which is an editor-specific plugin which contains custom code to connect the editor to the language server, but this part is meant to be a thin layer.
LSP (Language Server Protocol) a way for editors to talk to language tools to provide auto complete, warnings errors etc. this means the language tools can be written once and then any editor that has LSP support can use it https://en.m.wikipedia.org/wiki/Language_Server_Protocol
