As Exchange has progressed over the years, the roles have changed. Generally in larger environments you would have the Exchange 2010 roles like Mailbox and Client access each on their own servers, whether it was physical or virtual. The same would have applied to Exchange 2013.
Let’s take a look at the changes in each version:
Exchange 2010 Roles:
Mailbox Server
Client Access Server
Hub Transport Server
Edge Transport Server
Unified Messaging Server
Exchange 2013 Roles:
Mailbox Server
Client Access Server
Edge Transport Server
Exchange 2016 Roles:
Mailbox Server
In Exchange 2016, the roles have been consolidated, your CAS server is now a Service. Yes you read it correctly.
Below is some info on the Exchange 2016 CAS Service:
Exchange 2016 uses single building block that provides client access services & high availability architecture needed for any messaging environment at enterprise level. Client Access services provides authentication & proxy services on mailbox servers for client connections both internally and externally. To ensure that no data is queued or stored in them, Client Access services are stateless. These services are the part of the mailbox server, which is why it is not possible to configure Client Access Server individually as it was done in previous Exchange versions.
If you want to know more about the changes, you can click the link below to view the TechNet Article:
Hope it helps.