Google+ doesn't offer a search feature for messages yet, but it should be available soon. It's strange to see that Google had to disable a service to add support for a new site.
Search Engine Land also reports that Google's Wonder Wheel feature is no longer available. "[Google's] spokesperson said that the search tool was removed due to the 'initial stage' of the Google site redesign announced [last] week."
It's likely that both Google Realtime and Wonder Wheel had to be redesigned, but Google didn't want to delay the launch of the new interface until they're ready. When Google Instant was launched, many of the advanced features were not available, but some of them were added after a few weeks.
Update: The real explanation for shutting down Realtime Search is that Google's agreement with Twitter has expired, so Google no longer has access to the Firehose. "While we will not have access to this special feed from Twitter, information on Twitter that's publicly available to our crawlers will still be searchable and discoverable on Google," informed Google. That means Twitter will no longer have a privileged status and Realtime Search will become less useful. Microsoft and Yahoo still have access to Twitter's private APIs.
{ Thanks, Christian. }