I see where you’re coming from and it’s not often you would need a visited state for a button compared to a link but I think it’s still worth saying that you could have a visited state for a button.

For example, you could have a user flow where the user wants to change something earlier in the process/flow and then you may want to guide the user back to where they were which would mean having a visited state for the buttons they previously clicked on to reduce the cognitive load.