That had to be very distant past or some non-standard lisp. Common Lisp has no such feature.

That's a feature of the Lisp IDE.

File mode lines described language, the syntax (which Lisp dialect), the Package, the numeric base, the encoding, ...

Example:

    ;;; -*- Mode: lisp; Syntax: ANSI-Common-Lisp; Package: http; Base: 10 -*-

But to implement compiler directives in Common Lisp, this is not needed. That can already be done with macros and reader macros. If you for example use a macro in source code, the compiler will need to expand it. That allows arbitrary code execution during compilation.

It was done on Lisp Machines and currently in CCL.