MobileSafari doesn't seem to honour the disabled attribute on optgroups, see for example the example at https://developer.mozilla.org/en-US/docs/Web/HTML/Element/optgroup#Examples where the options in the third (disabled) optgroup are enabled in Safari on iOS.

Also, when optgroups are used, disabled options are still selectable, whether they are within an optgroup or outside of it. In the following examples the disabled option is rendered as disabled, but still selectable (and once selected, they are no longer rendered as being disabled):

<select>
  <optgroup label="My group">
    <option>My selectable option</option>
    <option disabled>My disabled option</option>
  </optgroup>
</select>

<select>
  <option>My selectable option</option>
  <option disabled>My disabled option</option>
  <optgroup label="My group" />
</select>

Tested using iOS 14.2 on iPhone 11 Pro and iOS 14.2, 13.7 and 12.4 in the iOS Simulator.