Varnish Purge client provided by package does not work when it comes to clearing all cache.
It should ideally expire / soft purge all cache in order to safely let cache be refreshed after a few requests against each page.
This is purely internal change at this point, the feature in question is not currently used, however it will in future release be used by possible cache clearing commands on command line for instance during deploy of changes.
Issue affects usage of PurgeClientInterface::purgeAll() when using Varnish. Symfony Proxy as well as Fastly already supports this.