Ship of Theseus still in use after hundreds of years! With long-lived sofware like this, I often wonder what fraction of the old source stays untouched for tha

Ship of Theseus still in use after hundreds of years! With long-lived sofware like this, I often wonder what fraction of the old source stays untouched for that long. At my day job, we have the distant descendant of a 35+ year old software package, but I can't believe there are many files which haven't had significant maintenance within the last 5 years. On the other hand, when I go looking I can typically find source files that have become totally unused and nobody seems to have noticed... Notable events in its history involve a semiautomatic translation from structured BASIC to to C in the late 80s, and finally getting under version control in the mid 90s. Also, I was somehow unaware of Macsyma being available on modern Linux systems under the name of Maxima so thanks very much for that!