symfony/symfony#29236

Looks like Symfony have decided to drop support for PSR-16 for some reason. We depend on \Psr\SimpleCache\CacheInterface for cache implementation, but they're deprecating support:

[NOTICE] 16384 'E_USER_DEPRECATED: The "Psr\SimpleCache\CacheInterface" / "cache.app.simple" service is deprecated since Symfony 4.3. Use "Symfony\Contracts\Cache\CacheInterface" / "cache.app" instead.' 

I think we have two options:

1. If Symfony are still going to ship the adapter, we can advise customers to wire the adapter themselves to Psr\SimpleCache\CacheInterface service
2. If not, we would have to include a third party PSR-6 to PSR-16 adapter