After a very tough number of months deciding upon a proper course of action, I ultimately concluded to end all forms of possible contact with one of my best friends since the 9th grade (for 11 years total). A number of things have contributed to the relationship's demise but none of them are definitive. That, the vagueness, is actually the worst part.
My hunch is that her current boyfriend doesn't want me around. At no point in time did we, my friend and I, have a fallout or any such issue. She simply stopped responding to any calls or texts in a reasonable fashion; everything devolving into one word answers or sentences with no substance. If there is/was a problem, I hoped she'd have spoken up. We've always been very open with each other and it's a sharp pain that it's ended up with naught but silence.
As much as I'm empathetic to people with extremely busy schedules, the "I'm really busy right now" angle doesn't hold up during the course of a year. If I can't get an hour or so for a drink/lunch here and there, I don't see myself as a part of her life anymore.
I wouldn't mind if she told me the reasons behind the cold shouldering. As long as I know what's going on I can deal with it and, inversely, being in the dark and being left behind is obviously not something that makes one feel valued. I can indeed understand if her boyfriend didn't want me around (it's pretty common for relationships anyhow), but I can't fathom any other explanations and it's digging at my nerves.
So, it's done. I've eliminated all forms of interaction, the biggest being Facebook since I still had to be privy to her posts and others linking to them, and I need to leave the history in the dust. 11 years is going to be extremely difficult to water down, but it's always one day at a time, right?