Would a butternut squash be a good substitute for the pumpkin?

Most canned pumpkin is actually squash, a variety very similar to butternut. As the linked article states, "all pumpkins are squash, but not all squash are pumpkins".
https://greatist.com/eat/canned-pumpkin-puree-is-actually-squash#pumpkins-vs-squash
I personally love using sweet potatoes, and prefer them to canned pumpkin.

Other ideas - any orange fleshed winter squash
Or even a sweet potato, if squash are getting rare in the markets.

Sure.

The recipe author covers this in the headnote: "Needless to say, pumpkin can be substituted with any of the earthy winter squashes."

My personal preference would likely be a Japanese kabocha squash but it's your dinner table, your guests, your choice.