At the moment, the two editors have feature parity and so support for both formats is possible, but this will inevitably be short term as we build on top of the new editor, meaning the Mobiledoc format will be dropped.
@Hannah – thank you for this reply! Quick question: when you say, "the Mobiledoc format will be dropped", do you mean it’ll be dropped from being an editor? Or do you mean even the Ghost rendering engine will at some point stop supporting Mobiledoc (which would necessitate all Mobiledoc posts actually converted into lexical with some kind of one kind conversion)?
I only ask bc I’m currently in the process of converting all of my lexical posts and pages back into Mobiledoc bc there’s currently a bug that prevents export/import of any lexical content. 
If you’re curious why we’re moving from one to the other, a quick look at the two linked repos should answer most questions!
I did take a quick look at the repos but could you please elaborate a bit on this? It’s true Mobiledoc appears to no longer be updated but it appears like it’s a mature, opensource product built by Bustle (last updated in Sep 2022; 44 outstanding issues). Lexical OTHO is built by Meta, and currently sports 246 issues. (I know the number of issues isn’t necessarily a proxy for quality but am still just curious.)
Thank you again for helping! 