Please see the discussion in
Future of IM (instant messaging) client in Zimbra and links from there.
Basically, yes, Zimbra's position is that "Beta" means it was never intended for production, and this seems to mean that removing it wasn't regarded as a regression. However, if you read everything that's been posted here and in bugzilla, I believe you'll find that Zimbra intends to continue to develop a new integrated IM client, and to provide hooks for third-party IM servers.
I accept that there may be good reasons for proceeding in this direction, but I think many would like Zimbra to provide a bundled IM server as an installation option. This is covered by
Bug 47036 - RFE: include separate IM server, so you might want to vote for it.