Zentyal-mailfilter is no longer available for community users. It is still available under commercial releases, though.
Spamassasin integration is done through AMAVIS, on MTA instead of on MDA, therefore such a plugin is not needed.
Finally, yes, HAM/SPAM accounts have been removed. You still can train spam heuristic filters by uploading a mailbox with spam entries thrugh Zentyal interface (or using sa-learn)