The only documentation of EuiComboBox options is derived from the public documentation site and from the Typescript definitions: https://elastic.github.io/eui/#/forms/combo-box

The typescript definitions indicate that each option is passed as HTML props to a button, which explains why the options are using HTML names like disabled instead of a name like isDisabled

{
  label: 'Titan',
  'data-test-subj': 'titanOption',
},
{
  label: 'Enceladus is disabled',
  disabled: true,
},
{
  label: 'Mimas',
},

Also, I see that groups are supported by structuring this as a nested array. It's unclear on how deep the grouping can go from the docs, but I assume it's up to one level.