Some people say it is because RTL8710 is not powerful enough (!?) to handle SSL. But in fact, you edit the gcc linker blacklist file to get around it. On Linux, the file is probably at
The original file contains:
rom_ssl_ram_map RTL8710 do not support SSL analogReadResolution RTL8710 do not support ADC analogRead RTL8710 do not support ADC
You can see that the RAM table referenced by SSL ROM is being blacklisted. Make a backup of the file and delete the first line.
Tried to compile WiFiSSLClient and it works properly on my RTL8710.