Is it possible to retrieve a users Skype for Business (Server) presence through EWS (on-prem), possibly if the Unified Contact Store is enabled? If so, where exactly can I find it?
No EWS doesn't provide any interfaces for Sykpe (or Teams) you need to use one of the Skype API's usually UCWA API is the easiest way to do this https://learn.microsoft.com/en-us/skype-sdk/ucwa/getpresencedataofacontact