The vhdl-mode maintainers need to look at your patch more closely; I just have some minor remarks.

7 maj 2023 kl. 00.11 skrev Cyril Arnould <cyril.arnould@outlook.com>:

> - I've added TYPE to the vhdl-compiler definition with the
>   appropriate choices for Info/Warning/Error and the dotted
>   pair. I'm not sure if sexp was the correct choice for the
>   dotted pair, is there a better alternative?

There is (cons ...) which would be more precise, see the manual.

The new doc string says that a TYPE of 2 is allowed but the type spec doesn't allow it.
Either allow both 2 and nil or change the docs to only mention one of them.

> - I added another entry to the backwards compatibility code, all
>   it took was a slight modification of the entry before
>   that.

That's fine, but I'd be a bit more careful with the destructive in-place changes and quoted list constants. (Think of what happens if later code performs an in-place change of that nil you added.)
This isn't performance-critical-code, we can afford consing here.