User blog comment:Kirkburn/Technical Update: April 13, 2015/@comment-452-20150418194704/@comment-166269-20150419015531

While I can't speak to plain-text emails, I will note that Wikia's HTML emails have used those tracking URLs for as long as I can remember; they're nothing new.

I have always been curious what the information in the URL decodes to, though: looking at it now, I can tell that any two-character string preceded by a "-" is a literal character escape (I see "-2B", "-2F", and "-3D"), but the rest I'm not sure of (Base64, maybe? but that doesn't seem right since there's only alphanumeric characters).