Listings
There are three 'lists' relevant to the format. There's the banlist, used for cards that players cannot make use of, despite being printed in Dominaria or prior. There's the whitelist, for cards that players can still play despite being printed after Dominaria.
There's also the watchlist, for cards that are in consideration for being banned or removed from the whitelist.
Some of these cards even have justifications behind their placement on those lists. These reasons are wholly opaque and will not be made clear to the public at this time.
Giant Leopards hasn't been printed yet, but it's watchlisted in case it ever does get printed.