I have previously wished for citeproc integration here:
There is some feedback from @KB on what may be needed to integrate. There are technical details that need to be addressed. All of the citeproc engines are not directly compatible with Obj-C, so a bridge is needed. in 2018 when I wrote that wishlist rs-citeproc was looking very promising but that project died. The most up-to-date citeproc engines are written in Javascript and Haskell. I suspect js-citeproc would be easiest to integrate as Apple does use JS via webkit and JSX already.
Now let me caveat: if you use Pandoc, you have citeproc built-in and the Scrivener+Pandoc workflow offers a fully functional citation workflow. The quickest fix would be to replace MMD with Pandoc and with some minor tweaks to the compiler, citations would be natively supported. The previous argument against replacing MMD with Pandoc was a packinging one, Pandoc is ~40MB where MMD is ~1.5MB
TLDR: I still fully support the request to have a citeproc engine as part of Scrivener, either via switching to Pandoc (with many other benefits), or using js-citeproc. I would’t use it personally as my Scrivener workflow already works perfectly with citations via and external Pandoc, but having it bundled would help less technical users.