Older C or C++ projects may be "standalone", but when built with the latest GCC or clang produce lots of warning messages and may be miscompiled courtesy of programmer vs optimiser. Older python programs may refuse to run under python3.
So while code does not rot, the GitHub project is unlikely to contain all the code for the project, and those external dependencies changing has much the same effect as rotting.
A still running CI would be a better indicator that the project works, but failing that, recent commits are the same signal.