I know that the theme review team has been active for about one year now. Thanks to their hard work and diligence, the themes in the repo are much better now than ever before. However, I have noticed that there are still older themes in the repo that, at first glance, probably would not pass the review if submitted or updated today. Is there a date that anyone can point to where we might want to be wary of any themes that were last updated before then? On a related note, is there a plan to possibly phase out some of these themes?
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.