Code in the book can easily be translated into less than 150 lines of code in a modern Lisp-like language to yield an interpreter, so someone somewhere probably

Code in the book can easily be translated into less than 150 lines of code in a modern Lisp-like language to yield an interpreter, so someone somewhere probably has a toy implementation. But almost certainly what implementations exist are not used to implement anything practical: most languages nowadays and especially most Lisps after the publication circa 1970 of the description of the Scheme programming language scope variables lexically (unless the programmer explicitly declares the variable to be dynamically scoped). There are a few Lisps that like 1.5 do not have lexically-scoped variables, e.g., Emacs Lisp before 10 years ago, but they dont suffer from the downward FUNARG problem that 1.5 has: in 1.5, if you specify an anonymous function as the argument to another function, you cannot specify any free variables in the anonymous function, which limits the expressiveness of 1.5. Or something like that. I am guessing that every one of the dozens of Lisps designed after the downward FUNARG problem was described does not suffer from the problem. In summary, the OP is an important milestone in the history of programming-language design, but the design itself is quite obsolete.