Describe the solution you'd like
EUI team has received a request to add a mixed state to EuiSelectable where options can be excluded.

/* Selectable option states */
unchecked | on | off | mixed (consumer app only)

Adding this new state will also require screen reader text to verbally communicate state being visually presented with the -. The new state should only be accessed by a consuming application. Users will still be able to toggle through unchecked, on, and off states.

Additional context
GitHub has a similar mixed state for issues when users select two or more using the row checkboxes. The discussion leading to this feature request referenced this action.