Is Salesforce Classic being deprecated?

For now, the plan is to switch everyone to Lightning by default (except for certain unsupported editions and license types).

The Classic UI will still be available:

After the critical update is activated, all users still have access to Salesforce Classic. Every week, starting the Sunday after Lightning Experience is turned on, Lightning Experience-enabled users who are working in Salesforce Classic are automatically logged into Lightning Experience. Users can switch back to Salesforce Classic as needed.

That said, Salesforce Classic has been "deprecated" for about two years now. Basically no new features are coming to Classic, there are many Lightning-only features already. Salesforce.com has already decided to move forward with the new framework rather than maintaining two different systems.