There have already been a number of users lamenting about the loss of this particular feature, especially since searching for communities using the communitysearch tool is less known about and, as such, most users don't know that it's even there.
http://www.livejournal.com/community/changelog/736406.html instituted this change, it appears. Is this a choice LJ has made - to not allow interest searching on communities by free users of the site? Can anyone speak to this issue, why this was chosen, and whether there are any plans to replace this functionality for free users?
(This issue was also mentioned in http://zilla.livejournal.org/1140 , as was brought to my attention.)
Currently, the answer being given to users is "We know this happened, we don't know why, and we don't know if it's coming back." It would be nice to at least give users a definite answer one way or another.