BPD (and other PDs as well) consists of a spectrum of behaviors. Considering that it's a disorder, then we're talking about poor behaviors.
Some leave and never come back, as may be the case with you. Others may seem to leave but actually have you "still on the stove but simmering on the back burner". You may not be able to determine that until the ex's next relationship or two fails and you get contacted again.
With some relationships, the ones with children, it may never be totally over due to the necessary child-related communications.
So be aware that if you're surprised in a few months or years with renewed contact, then you need to be resolved not to repeat the past. (Well, unless you're very confident your ex has sought long-term therapy and diligently applied it in life and thinking. Generally unlikely.) As in, once burned twice shy. Another is this:
This has been said so many times that I don't know if there is an original quote to cite, but I remember in the original Star Trek series the Enterprise's engineer Scotty finally exclaimed to those on the Bridge, speaking of the Klingons trying to keep luring the Enterprise away from Captain Kirk and the landing party with a second false distress signal, "Fool me once, shame on you. Fool me twice, shame on me!"
There is no easy way to get closure from a seriously disordered ex. So it is best to Gift yourself Closure and move on.