Not to me it isn't. If possible a break-up should be in person or at least on the phone. Don't text ! Call at least!
Is a break-up via text a legitimate break-up?
It's a dumb way to break up, but yes.
The least the guy could do is call. I mean, if you want to talk in technichal terms I'd say yes, it is a break up but it is the dumbest way to break up with someone.
Well, I wouldn't say it's among the best ways of doing it. I know that sometimes it might be scary to look at a person in the eyes and tell them it's over, but it is always good to let your feelings out and it is also a nice way to show delicacy and respect towards the other person.
Although breaking up via text isn't the best idea, there are some situations where this might be necessary. Maybe this person is avoiding you, so the only way to tell them is through a text. Maybe it's safer to text them because this person is abusive and you don't want to take any risks. Here you can find a list of 10 times it's okay to dump someone via text.
However, unless the situation is extreme, I would recommend to take up all the courage you have and tell them in person. It will be better for him or her, but it will help you as well: It's a good chance to grow and learn how to express your feelings. Best of luck!