It might not be full second class but I dunno I'd go all the way to first class. For example F# doesn't support the source generators feature that came with .NET 5 (unless they added it in 6 and I missed it but when I looked at that post you linked I didn't recall seeing it).
They got type generators which somewhat influenced and predates the Source Generator feature by a few years. Not the same thing of course, and from what I've seen are slightly different in target/scope. (e.g. F#'s seems more appropriate for scripting where you can use it without a proj entry).
Do you mean type providers? That's their own type of nice thing but to your point it is only for generating types, source generators can generate much more than a basic type. Also I swear I saw somewhere type providers were not functioning as well in newer .NET but that was hearsay as I never tried them in 5 and now 6.
They are working in .NET 5 last time I checked. Opened an FSX script, "#r: nuget SwaggerProvider", point to a JSON API and it seems to work with IDE auto complete included with 2 lines of code, no project scaffolding, or package manager files required. Things seem to be improving on this front I guess as well.
I do agree - source generators have a better API as an implementer and can generate more than classes. I guess as soon as I need to jump into a proj file and I'm working with MsBuild anyway what stops me creating my own generation framework that runs before PreBuild that generates code for my compile step? I've done it before source generators were a thing.
I also did a quick Google search - Myriad (F# library) might be something that does something similar.
