That's interesting. Per the explanation on the FCC page, it's so all numbers that start with 988 go to the 988 line. I guess they want to catch misdials by callers who need the service, but are only able to catch one class of misdial (extraneous trailing digits) without requiring thousands of people to change their numbers. There were obviously some hard conversations behind that choice.
It's not just trailing digits, if you don't have a call button (like a traditional home phone, push button, or rotary) you need to know when the dialing has finished so that you can make the cell call