This is a new feature added in LSP 3.17.
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_inlayHint

Inlay hints allow the editor to display virtual text containing some additional information, for example parameter names:

TTCN3 supports named parameters but they are optional, and this feature could nicely complement them.

Is there a plan to implement this in ntt? I played around a bit and got a PoC implementation (just CallExpr/FuncDecl for now), I can send a PR if there is a chance it could be accepted.