No, TMI slang isnt limited to texting or online chats. People also say it in face-to-face conversations when someone gives too many details. For example, if a coworker starts describing their surgery in-depth during lunch, another person might say, Okay, TMI! Lets change the subject. While its more common in online communication, its widely used in everyday speech as well. So overall, there is no matter where you use online or offline you just know the meaning and uses.