My project and the referenced primer are about the 1976 standard.

But many MUMPS based systems are still in operation and maintenance today; and not many significant systems in IT reach a 50 year lifetime.

A modern JavaScript, PHP or Python system (languages with similar limitations for large-scale software engineering as MUMPS) written five years ago hardly works today because dependencies significantly changed or are no longer available. In 50 years (or even in 10) it will be astronomically expensive to keep a current Node.js system alive. But you still can run an unmodified 1985 MUMPS system on a current InterSystems IRIS server.

The main problem with critical MUMPS systems today is less technical, but mostly staff shortage. The same applies to COBOL, or Ada, or even Java.