This is- means, presently she did this in the past. It is true to. Right now, she did do that in the past. It will always be the case. She can not change now, or ever, what she did. This was what she did- has the same idea except it does not point out that it is true now. It is true now, that she....... but it does not stress it. It tries to leave that in the past.

Excellent question. I say that either is acceptable. I suspect that you will often find the former when the "This" is examined in detail, in the present tense. Example: "This is what she did: she shaved off all of her hair, and changed into a monk's clothing, because this disguise is unremarkable and common in that part of the city"