Does your Ada based project suffer from code rot when the Ada compiler isn't injected with fresh code from a living developer?
No, but performance will decline relative to other programming languages that have actively developed compilers. Take the case of Common Lisp's CMUCL and SBCL. Although still actively developed, development is slower than in their earlier years when their performance nearly matched C. Now the gap between SBCL and GCC's C compiler is widening because GCC's C compiler is far more actively developed.
Of course, my Ada programs will still work even if GNAT is no longer developed. However, I will be missing out on future speed improvements relative other languages.