it's a fairly common usage in numeric computing. If you read, for example, the wikipedia entries for "computational fluid dynamics" you'll see that they consistently speak of "codes" when referring to programs.

https://en.wikipedia.org/wiki/Computational_fluid_dynamics

Same for computational Chemistry, computational Physics (solid state, neutronics, astrophysics), etc.

It's because the types of things people write in Fortran (high performance science codes, for example) tend to be monolithic, single-purpose programs. It comes from a time when a code really was basically one compilation unit (and doing that is such a nice simplification that I support it, for science). With code written for the web, shared through package managers, etc. it makes more sense to use the uncountable noun instead of the countable one.

I've never seen the word used in the plural (for computer programming code) until today, having been in computer science for 35 years. The uncountable form definitely dates from way before the web.

As someone doing research in physics, I’ve noticed this usage before. It seems fairly frequent outside CS [EDIT: and as the sibling comment says, specifically in numerical computing]. From what I’ve gathered, for them ‘code’ has become a count noun, such that ‘a code’ means something like ‘a piece of code’ or even ‘a program’, and the plural ‘codes’ follows from that.

I'm seeing it here for the first time, and from multiple people. Maybe it's specific to the Fortran community?

Yes, in some traditional Fortran environments "codes" has been used where in other environments one would have said "libraries" or "programs", e.g. "these are some codes for solving systems of ordinary differential equations" or "this is a code for solving boundary elements problems".

So in this sense, as a synonym for a library or a program that accomplishes some function, "code" is countable.

It’s not, but it’s somewhat common among people making numerical models of dynamic systems and that sort of thing. People like Steve Brunton will often say all the “codes” are available to go along with his videos for example https://youtube.com/@eigensteve?si=IVLparAOZ9XDauTz

Yeah, I figure there's a large overlap between those two groups.

I think "codes" is peculiar to what is now called HPC, where it's been common since at least the 70's. It's not specific to Fortran; there are C/C++ HPC codes too.