[koito17]

> Lisp traditionally has immutable data structures

Can you provide a source for this? Common Lisp and its predecessors certainly do not have immutable data structures out of the box. The usual data structures are cons cells, lists (composed of cons cells anyway), vectors, and hash tables.

> I would be very interested if it’s possible to create an extremely simple garbage collector or an ergonomic Lisp that isn’t garbage collected, but a solution doesn’t spring to mind.

PreScheme immediately comes to mind for me as does ulisp. The latter in particular is designed for microcontrollers with limited memory.