See David R. Hanson's "A Portable File Directory System" [0][1], for example: a 700 lines long implementation of early UNIX's filesystem API that piggy-backs on some sort of pre-existing (block-oriented) I/O primitives, which means you can do it entirely in-memory, with about another 300 lines of code or so.

I suspect that with OSes becoming much more UNIX-like the demand for such abstraction layers shrank almost to nothing.

[0] https://drh.github.io/documents/pds-spe.pdf

[1] https://drh.github.io/documents/pds.pdf