Hello Iuri,
I am no XML-RPC expert, but from some brief search I could find that the original website for the validator is still there: http://www.xmlrpc.com/. However, seems like all the relevant links are dead... maybe you could try contacting the people that are paying for that website and ask them what is happening...
About the one for Worpress, this is a link from the author explaining why he came out with the idea https://daniloercoli.com/2012/05/15/wordpress-xml-rpc-endpoint-validator/
Probably more interesting, this is the code of the thing on Github https://github.com/daniloercoli/WordPress-XML-RPC-Validator
If you see the readme, seems like the validator works in two steps. First is quite generic and -should- apply to any implementation (as far as I understand). Second step seems more Wordpress-specific, as it looks for a user profile, uploads stuff etc.
My two cents are to first see if the original, or equivalent validator is still accessible somewhere, as website or source, otherwise you could either fiddle with the one for wordpress, or use it as blueprints to build one from scratch (of course only for the generic part).
XML-RPC is an interesting feature IMO, that could be nice revamping. If you're embarking on this I could give some support.
All the best
Antonio
