Subtitling is one of the most commonly seen accessibility features, with some publishers (such as Ubisoft) even requiring it. It benefits a huge range of players, not only those with permanent hearing impairments, but also if the spoken language is not your own first language, and situational impairments such as playing in a noisy environment, through poor quality hardware, or having to play on mute to avoid waking the baby.
However, it is rarely done well. Ensure that speech in all areas of the game – gameplay, cutscenes, menus – is covered, and also ensure that the subtitles themselves are formatted well.
Best practice example: Dead to Rights Retribution