It is being developed, just under a new plugin title.
WPSSO Core was forked from NextGEN Facebook back in 2014, so initially, yes, they shared much of the same code. And even early on, in 2015 and 2016, a few features and code improvements from WPSSO Core were back-ported to NextGEN Facebook. Aside from a few contributions from the WPSSO Core code, NextGEN Facebook itself hasn’t seen much development since that time. Since WPSSO Core and NextGEN Facebook handle settings in much the same way, WPSSO Core is probably the easiest option to migrate off of NextGEN Facebook, although if you don’t mind re-entering some settings, Yoast SEO and The SEO Framework are also good alternatives (though they lack some of the more advanced content features and Schema support of WPSSO Core).
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