For the deploy and upgradecharm commands, when a series is not provided, and a default-series is not set in the environment config, the client will resolve the series with the charm store, through the state server. If the client is working with a 1.16 state server, it will resolve the series directly with the charm store.
Existing environments will have a default-series, so they should have no change in series selection.
New environments will continue to support the default-series config setting, but it can be omitted, and is not set by default.