That certainly might be one of the oldest Fortran codebases still in use I've heard of, but not quite the largest. (Character came in '77 wasn't it?)

E3SM [0] is still maintained today, and somewhere around 2mil LoC of F90.

Modern Fortran has a lot of niceties that it brings to the table.

[0] https://e3sm.atlassian.net/wiki/spaces/DOC/overview | https://github.com/E3SM-Project/E3SM