SyslogAppender for log4esp
Class Print is a base class of Arduino streaming. Stream is a derived class and adds reading capability. Print has only one abstract method write to write one byte to output or buffer. It is easy to implement a derived custom class.

The classes derived from Stream are classes for hardware and software serial like HardwareSerial for Serial singletons and different software serial implementations. From Stream is derived class File of the SD and SPIFFS library, TwoWire class for Wire singleton object, etc. 

Class Client is derived from class Stream and is a base class for Arduino standard TCP networking. Examples of derived classes are EthernetClient, WiFiClient. For UDP, class UDP is derived from Stream, of course with implementations like EthernetUDP, WiFiUDP.

You can see this in the source code in local source file folders or in GitHub repositories.