Block settings vertical tabs only have summaries for core conditions

> Summaries are a javascript only implementation, there is no server roundtrip. Whatever we do, there is no way around each condition that wants to display a summary having to provide a javascript snippet for this.

I don't think that's true. The JS that creates a summary line is completely generic: it just takes the checkbox labels and joins them together:

      function checkboxesSummary(context) {
        const values = [];
        const $checkboxes = $(context).find(
          'input[type="checkbox"]:checked + label',
        );
        const il = $checkboxes.length;
        for (let i = 0; i < il; i++) {
          values.push($($checkboxes[i]).html());
        }
        if (!values.length) {
          values.push(Drupal.t('Not restricted'));
        }
        return values.join(', ');
      }

That would cover most condition plugins in contrib.

The only thing that needs to be changed would be the JS that decides which vertical tabs to apply the above to:

      $(
        '[data-drupal-selector="edit-visibility-node-type"], [data-drupal-selector="edit-visibility-entity-bundlenode"], [data-drupal-selector="edit-visibility-language"], [data-drupal-selector="edit-visibility-user-role"], [data-drupal-selector="edit-visibility-response-status"]',
      ).drupalSetSummary(checkboxesSummary);

Make it opt-out so more complex conditions that supply their own JS and declare that they're opting out in their plugin definition.

I call this a bug -- Drupal is meant to be extensible, and this isn't.

I agree and would also vote for bug, given that there's even a place in the condition plugin's code where you provide a dynamic summary to be picked up in this exact vertical tabs place.