I think you may be missing the point. Metafonts have absolutely nothing to do with "variable fonts". It's a completely different branch of font technology. You cannot produce a variable font as OP intends using metafont.
metafonts, as implemented by the metafont program, and variable fonts are literally exactly the same thing: a parameterized font which produces an infinite set of fonts as you vary some parameters
i think what you mean to say is that the metafont program isn't compatible with truetype or opentype, but i kind of already explained that in the comment you are replying to
that page opens by saying it's a generic term, not the specific implementation of the concept in opentype:
> A variable font (VF) is a font file that is able to store a continuous range of design variants.
this definition is the same thing as the definition of a metafont, and clearly includes the metafonts implemented by the program named metafont. it seems like the authors of the page didn't know about the program named metafont, though, so they mistakenly think apple invented it
It does not claim it's a generic term. The whole page is about variable fonts as derived from "Apple's TrueType GX font variations"; one sentence taken out of context doesn't change that. If you search the internet for variable fonts, you only find people talking about this technology. Parameterized fonts as in metafont have never been called variable fonts and trying to claim otherwise is just creating unnecessary confusion.
