I guess your switches have gigabit fiber ports (GBIS or SFP). They are not using SONET. It is a totally different technology. However as I know 4500 and 6500 series switches may have SONET port adapters.
Take a look at this link for more info about SONET
Honestly I don't know whether SONET is being developed further. The SONET and SDH has been widely deployed in telco networks and obviously have advantages and disadvantages. It is a quite expensive technology so nowadays the providers tends to deploy Metro/DWDM/MPLS network. If you plan to connect to switches back to back the easiest is to use Ethernet.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...