I’m not sure, the information on this says that WP will store the spit terms, I’m guessing in the options table, but I don’t know for sure.
There is a hook that’s fired when a term is split so that plugins can correct.
This was the first announcment https://make.wordpress.org/core/2015/02/16/taxonomy-term-splitting-in-4-2-a-developer-guide/
and this was the second https://make.wordpress.org/core/2015/06/09/eliminating-shared-taxonomy-terms-in-wordpress-4-3/
WP considered shared terms a edge case and basically left it up to the developers.
