The menuitem, menuitemcheckbox, and menuitemradio descriptions say the following:

Source: https://w3c.github.io/aria/#menuitem
Authors MAY separate menu items into sets by use of a separator or an element with an equivalent role from the native markup language.

While it isn't explicit, presumably this means that separator should be in the list of required owned elements for menu. Happy to open a PR for this is if people think this is right.