This isn't part of the current discussion, but what is the appeal of single-header libraries?

Most times they actually are a normal .c/.h combo, but the implementation was moved to the "header" file and is simply only exposed by defining some macro. When it is actually a like a single file, that can be included multiple times, there is still code in it, so it is only a header file in name.

What is the big deal in actually using the convention like it is intended to and name the file containing the code *.c ? If is intended to only be included this can be still done.

> avoiding the C standard library are regressive and obsolete

I don't understand this as well, since the one half of libc are syscall wrappers and the other half are primitives which the compiler will use to replace your hand-rolled versions anyway. But this is not harming anyone and picking a good "core" library will probably make your code more consistent and readable.

With just a single file you can trivially use it such that everything is inlined (if it's of the sort that static-s all functions, at least), even across multiple files using it, without needing the full compile-time-destruction of LTO.

And generally it's one less file to look at, more easy to copy-paste into your project (and as a very minor security benefit you'll potentially look at arbitrary subsets of the contents every time you do a go-to-definition or use the header as docs (thus having chances to notice oddities) instead of just looking at a header).