... but just filter or reduce the keys?

Yup, that's what I did 🙂

I'm finding malli very useful for client and server side entity validation. When you have an entity that a client can submit, but is only allowed to submit a subset of all possible attributes and the other attributes are populated by the server. In a ns for an entity, I have one main custom registry (which is just a map!) that defines all possible attributes of the entity, then I build little schemas that pick out the "views" of what a valid entity looks like at different stages in its lifecycle.

What's the story behind the name malli, is there one?

https://www.slideshare.net/metosin/malli-inside-datadriven-schemas#21 (slide 21)