The identity of the new Super Smash Bros. Ultimate DLC characters is normally kept a secret to Kirby's voice actress, but she managed to discover Terry's identity based on her experiences with Fatal Fury.
One of the considerations that have to be made whenever a new character is added to Super Smash Bros. Ultimate is Kirby, as his Copy Ability allows him to steal the appearance of his enemies - along with one of their attacks. Kirby can copy every character in Super Smash Bros. Ultimate, with his most recent power-up giving him the Power Wave if he manages to use Inhale on Terry Bogard.
Kirby's voice actress is Makiko Ohmoto. She's brought in to voice new lines whenever a character is added to Super Smash Bros. Ultimate, depending on if their new move causes Kirby to speak. Ohmoto is never told who the character is in order to maintain the secret for as long as possible. According to Japanese Nintendo, Ohmoto said in an interview that she worked out Terry's identity from Kirby's new dialogue.
Ohmoto said that the only two fighting games that she had played were Street Fighter 2 and Fatal Fury, as her younger brother was a huge fan of them. The producers would never tell Ohmoto which character she was voicing, but she worked out Terry's identity based on Kirby's shouts. Terry never shuts up in the Fatal Fury & The King of Fighters series and always shouts the names of his special moves when using them, which is why Ohmoto recognized them when performing the lines as Kirby.
It's a recurring element of fighting games for playable characters to shout out the names of their attacks as they perform them, which is why so many people remember "Hadoken" and "Shoryuken" from Street Fighter, as well as Captain Falcon's "Falcon Punch" in the Super Smash Bros. series. Terry's shouts have become infamous for their poor English and they went on to reach memetic status in the SNK fan community. It seems that Terry's shouts left an impression on Ohmoto if she still remembered them after so many years.
Terry Bogard is available now as a DLC character in Super Smash Bros. Ultimate.
Source: Japanese Nintendo