What do you think about introducing a new base class for servers? One based on what is actually useful. It would be more a guide than a useful base class, but for Client and UDP the base classes do an excellent job for consistency of the networking API implementations.

template <class TClient>class ServerNew {  virtual void begin(uint16_t port) = 0;  virtual void end() = 0;  virtual explicit operator bool() = 0;  virtual TClient accept() = 0;};#define CORE_HAS_SERVERNEW

It doesn't require the confusingavailable method and the never used print-to-all-clients' functionality.
It has parameter port forbegin, it doesn't have to be in constructor.

The library authors could ifdef their server class declaration.

class EthernetServer #ifdef CORE_HAS_SERVERNEW   :  public ServerNew<EthernetClient>#endif{

or even

class EthernetServer :#ifdef CORE_HAS_SERVERNEW    public ServerNew<EthernetClient>,#endif    public Server{

would work

Here is an overview of Server implementation in libraries
https://github.com/JAndrassy/Arduino-Networking-API/blob/main/ArduinoNetAPILibs.md#server-class