I do not think it is feasible to maintain all language versions in parallel for eternity. What would make porting this code to a newer standard difficult in this specific case?
It's a good thing to only have declarations in headers, it simplifies parsing with 3rd party tools (to generate language bindings), and speeds up compilation (see the C++ stdlib headers for the canonical "why is implementation code in headers a bad idea" example). IMHO mixing interface declarations with implementation details was one of the cardinal sins of C++.
> What would make porting this code to a newer standard difficult in this specific case?
It's pointless busywork, and it's unclear how deep the changes should go (for instance: does it make sense to move C89 variable declaration from the start of scope blocks to their initialization point just because it might potentially be "safer" but touches half of all lines of code?).
The code demonstrably works as C89, and any bugs that matter most likely had already been fixed decades ago, also doing large scale syntax changes is just noise in version control which makes bug fixing harder (because that often involves diving into the change history).
Compiling with a new language mode does not force you to move variable declarations to the point of initialization, but you would now have the option to improve the code in this way when it makes sense. It is difficult to see this as an disadvantage. My point is that moving it to a new language version would usually not require changes to the code, except where the old modes were dangerous. So I would expect to find serious bugs when doing this.
That's actually a good point hmmm (not in case of this specific assembler, which seems to be quite robust and well-written), and even if there would be serious memory corruption errors lurking, they would be contained because the assemblers runs in a WASM VM.
For projects that are still maintained, a "moving standard" isn't that much of a problem, after all we've been conditioned that every new compiler update adds new warnings which "break" existing code ;)
...there is a certain value in taking a "finished/frozen" project and integrate that into a new project without requiring code changes though.
I'm sure there must be a good middle-ground for C, where obviously bad and outdated language features (starting with leftovers from the K&R era) can be removed without causing too much breakage even on most old code ... but then, why keep 'inline' ;P
...rather, doesn't need. C++ modules just (potentially at least) fix a problem that C++ created in the first place (exploding build times because of complex template code in stdlib headers).
Also, I have yet to see clear indications that C++ modules drastically improve build times in real world projects.
The trick to make them faster is the same as with C++, never compile everything from source unless required, this includes template code for common type parameters.
As for compile times see Microsoft Office modules migration, or that VC++ import std in C++23 is faster than a plain #include <iostream>.