Maybe "Historical Isekai"?
I certainly wouldn't group it with most other isekai that make up the broad majority of the "genre", but I think the term can be applied broadly to many settings in which a character finds themselves well outside their usual life and comfort zone via sudden transportation of some sort.
I'd almost want to actually tag more stuff along these lines like this as isekai so that it might be discovered by those who look specifically for Isekai stories and might discover something of higher quality than they might otherwise.