ITC Stone exists in both Sans and Serif options, and Lucida Sans is paired with Lucida Bright (serif) as well as Lucida Fax (almost slab serif).

Adobe's Penumbra font has a sans--serif axis, well, had.

Similarly there was the font Walker which had "snap-on" serifs:

https://www.moma.org/collection/works/139311

Yes, the distinction is quite subtle for the variable axis options.