How long has the Magus Undead Rescuer been available from rare souls?

[Amarthir]

I just pulled him from a rare soul. I thought he was still "exclusive". Did they sneak him in unannounced?

[Yup]

There's no such thing as "exclusive" in the game....characters appear to just be only available one way for generally the first week.. that's all. It would seem that the closer it is to the next monthly game update, the more likely the "exclusive" character will be added to the standard Rare/Legendary souls.

[Kardas]

I just pulled him from a rare soul. I thought he was still "exclusive". Did they sneak him in unannounced?

They did what now

You're right though. In the fragments screen he's listed as a drop for both the Rare and Legendary Soul (didn't bother checking the Rare Magus Soul).
I also checked the Game Update posts, but didn't see it there either. I think you're right in that they forgot to announce it.
Thanks for letting me know though! Will update my Souls thread as a result.