I have compiled Fortran programs from the 70s on modern platforms without changing a line. The compiler, OS, and CPU architecture had all disappeared but the programs still worked correctly.
I've 'modernised' such an old program then come back a few years later, realised the 'modern' ones don't run anymore and reverted to the old Fortran version.
That is likely a compiler issue. Besides being modern, Fortran 2018, 2008, 2003 have no backward incompatibility issues.
You misunderstand. I 'modernized' to Python (similar to the GP). Then the Python version stopped being supported so I reverted to Fortran..