That question is been solved, as Pokemon have been verified to retain their STAB. A Charizard, for example, possesses both Fire and Flying STABs, which offer a 1.5x advantage to Fire or Flying-type attacks. When a Charizard gets Terastallized with a Water Tera Type, it retains its Fire and Flying benefits while also gaining a Water bonus. If Charizard obtains the Fire Tera Type, its Fire STAB will rise while their Flying STAB will remain same.
It should be noted that defensive typing benefits are the inverse. A Charizard is vulnerable to Electric, Rock, and Water since those kinds are naturally powerful against Fire or Water. If that Charizard Terastallizes with a Water Tera Type, they will instead be vulnerable to Water, Grass, and Dragon. They will lose their inherent vulnerabilities and inherit the Tera Type’s deficiencies.
