For an exposed Boolean filter on the Order shipped field for the purchase page of the transactions view of a custom entity in my rsc module in Drupal 8, I used the following. Note that I had to add _value to the field name.

function rsc_form_views_exposed_form_alter(&$form, &$form_state)
{
  if ($form['#id'] == 'views-exposed-form-transactions-purchase') {
    $form['field_order_shipped_value']['#options']['All'] = 'Any';
    $form['field_order_shipped_value']['#options']['1'] = 'Yes';
    $form['field_order_shipped_value']['#options']['0'] = 'No';
  }
}

Concrete example of an exposed Boolean filter for a view on a custom entity. Note that I had to add _value to field names.

• Custom module name: rsc
• View machine name: transactions
• Page machine name: purchase
• Field name: field_order_shipped

Actual code:

function rsc_form_views_exposed_form_alter(&$form, &$form_state)
{
  if ($form['#id'] == 'views-exposed-form-transactions-purchase') {
    $form['field_order_shipped_value']['#options']['All'] = 'Any';
    $form['field_order_shipped_value']['#options']['1'] = 'Yes';
    $form['field_order_shipped_value']['#options']['0'] = 'No';
  }
}