All things come to an end, and extensions aren't an exception. J4Schema was my first extension, created back in 2012; it was a very interesting experience and it taught me a lot about Joomla environment, however now it's completely obsolete.
In these years Google changed its algorithms and adding all those microdata to your pages is no more useful: Google is now clever enough to extract the interesting bits from your pages and there's no need for an external tool.
So, once again, farewell J4Schema! You were an amazing project, but now your time has come.