drush smart_date:migrate / The "" entity type does not exist.

Aha. It works as expected when --entity=node is specified in the drush command, like so:

$ drush smart_date:migrate event_program field_when field_event_date --entity=node

Smart Date 3.5.0 declares a default value for 'entity' in SmartDateDrushCommands.php, differently than in prior versions:

public function migrate($bundle, $dest, $source_start, $source_end = NULL, $source_all_day = NULL, array $options = NULL) {
    if (!$options) {
      $options = [
        'clear' => FALSE,
        'entity' => 'node',
        'default_duration' => 0,
        'langcode' => NULL,
      ];
    }

The check on $options seems to be failing here(?).

Smart Date 3.4.3 has:
public function migrate($bundle, $dest, $source_start, $source_end = NULL, $source_all_day = NULL, $options = ['clear' => FALSE, 'entity' => 'node', 'default_duration' => 0, 'langcode' => NULL]) {

After updating to Drupal core 9.3.5, the workaround of specifying the entity value in the drush command no longer works.

$ drush smart_date:migrate event_program field_when field_event_date --entity=node

Gives the error, 'The "--entity" option does not accept a value.'

Rolling back to Smart_date 3.4.3, the drush command still works.