Compatibility with Purge/Acquia Purge?

There are no current plans. However, this is a good idea. If a MR with this feature is introduced I would be happy to include it in the next release.

It looks like the alerts response is already using the `sitewide_alert_list` cache tag and has the module's settings as a cache dependency.

See Drupal\sitewide_alert\Controller\SitewideAlertsController:load(), ~line 104 → :

// Set response cache so it's invalidated whenever alerts get updated, or
// settings are changed.
$cacheableMetadata = (new CacheableMetadata())
  ->addCacheContexts(['languages'])
  ->setCacheTags(['sitewide_alert_list']);
$cacheableMetadata->addCacheableDependency($sitewideAlertSettings);

$response->addCacheableDependency($cacheableMetadata);

In my testing, this is respected and the cache for this route is invalidated as soon as an alert is created/updated/deleted. Which means that I'm able to configure the module's Browser and Shared Cache Maximum Age (in seconds) setting to an extraordinarily high number and still seem updates reflected immediately.

I wonder if it makes sense to add a new setting to the module's setting form. Something like:

Use Drupal's Browser and proxy cache maximum age? with a default value of yes
If no, expose the form as it exists now.

We'd also need to alter Drupal\sitewide_alert\Controller\SitewideAlertsController::load(), ~line 127 → to respect this new setting.

And then for BC, we'd need an update hook to set appropriately in existing sites, i.e. to not use Drupal's global max-age.

Thoughts?