What is the general cut-off date for reviewed themes in the WordPress.org repository?

Anything updated since late 2010 or so should be pretty reliable. The Guidelines have been pretty much stable since fall 2010 or so.

As for existing Themes in the repository: I think most of the Theme Review Team would like to see some policy implemented in which obsolete Themes could be suspended; however, that decision is above our pay grade. For the time being: any Theme already in the repository, regardless of when it was approved, will remain active/available.

We have removed some older Themes, if they were found to have a security exploit, malicious code, non-GPL-compatible license, or spammy links.