I think you guys are confusing Master Slytherin with Mrriddler, who lost interest in HP fanfiction quite a long time ago.

 

So it's confirmed that he's lost interest in the story.

Mrriddler still posts on FF.net: last story update was in Feb'. You think he'll let someone adopt it, if they asked?

Edit: I sent a private message to him over fanfiction.net, asking if the story was up for adoption.
I received a very quick reply, but unfortunately, it's bad news:

I understand his viewpoint, and can even agree with it. Still, it's a shame.