Currently we don't support stale HTTPCache, aka in Varnish terms: grace.

This is because we currently use invalidation strategy (Purge/Ban), as opposed to expiry strategy, where you would have stale (and potentially outdated) cache.

Tech notes (might be invalid)

Invalidation

In Varnish there might be solution only available in Varnish Plus, however can not find any doc on this besides issue with patch: softban

In FOSHTTPCache this can probably be solved as well, however needs development.

Expiry

Alternative is to somehow go back to expiry, however keeping track of expired content in (a layer similar to) Symfony Proxy, using Stash for the info for fast If-Modified-Since responses.