Where did you get the idea that Windows Defender was being phased out?
Windows Defender is included in Vista and Windows 7 as part of the OS.
It can be downloaded for XP as an add-on.
Microsoft Security Essentials is an antimalware product that includes
the protection of Defender, but expands upon it. It is completely
optional. One is free to use any Security product that they desire for
protection against malware and may use Defender, too. Some other
antimalware products also disable Defender, just as MSE does, to avoid
conflicts.
In any event, I'm not aware of a specific forum that they plan to
direct people to for discussion of Defender should they choose to shut
down these newsgroups, too, but you can always post to the Answers
forums for Security related to the operating system in use.
-steve