Hi,
here is my two cents am sure Zhang knows more :
first of all it is very odd this warning occurs if i am correct you had dovecot already installed before installing iredmail. Iredmail expects a clean system so it can perform a nice very good working system install. This is because there are so many versions out there of everything as is the case with dovecot. Your version of dovecot is different therefor you will need to make manual changes to the dovecot.conf file. (or ignore the warnings as it still works just mentions this warning in the log as the function is deprecated and will be removed later)
the function mentioned that is deprecated as of 2.1+
http://wiki2.dovecot.org/Plugins/Autocreate
The entries to add to dovecot.conf
http://wiki2.dovecot.org/MailboxSettings
Also all references to auto-create must be removed or commented out.
(autocreate removed form mail_plugins and
find autocreate entries and comment them out as below.
# Plugin: autocreate. Create and subscribe to default IMAP folders.
#autocreate = INBOX
#autocreate2 = Sent
#autocreate3 = Trash
#autocreate4 = Drafts
#autocreate5 = Junk
#autosubscribe = INBOX
#autosubscribe2 = Sent
#autosubscribe3 = Trash
#autosubscribe4 = Drafts
#autosubscribe5 = Junk
However I highly suggest to do a clean build and build iredmail as it should to avoid more issues and make it easily to support.
just my two cents 
regards,