Cache backends are now defined in the DIC.
To have something available in the DIC as a service, it needs to be registered in a services.yml
Only modules can provide services.yml
Previously, you could enable memcache (or another session/cache/lock/queue backend) entirely via settings.php without having to enable a module at all. Now the service would have to be defined services.yml in memcache module, then settings.php could do the rest.
This affects both large multi-sites where you want to configure every site to use memcache just once (via a shared settings.php or whatever), and also potentially dev/staging where services might want to be different based on the environment.
Not sure what to do about this, when discussing with Damian we thought about sites/*/services.yml (and/or allow settings.php to specify a file location).