I agree that the wording doesn’t convey unambiguously.
My metaphor for the keymaps is like a kids picture book with transparent overlay pages, but of course that fails because with a keymap stack you can apply each layer independently without the ones below it, while in a picture book if you are looking at layer 3 then layers 1 and 2 are also active.
A metaphor of a stack of layers with toggle switches would more closely reflect the design than my picture book, so I see the merit of “Toggle.”
However, the metaphor is a bit weak it the sense that it doesn’t evoke a physical analog, and so isn’t viscerally compelling for folks. No one is likely to have an “aha” moment about toggle switches and layers. It’s more correct but less evocative.
I will noodle. What comes immediately to mind is Russian dolls, with the inner doll being zero and the ability to surround zero with an arbitrary set of larger dolls in a fixed order, but adding the third dimension muddles things.
I can imagine transparencies on a pivot that can be rotated in and out independently, but can’t think of a real world object that works that way. Has to be something.