I thought I had enabled that – check out the doveconf -n listing.  Did I miss something?

IMAP METADATA for user-defined imapsieve scripts would be useful to you if you have clients that support that. If you know of any, please do share.

Mar 19 16:21:48 mhv3 dovecot[47532]: imap(steve)<47541><ROnt+uu9idkYgsde>: Debug: imapsieve: mailbox INBOX: Mailbox attribute /shared/imapsieve/script not found

Mar 19 16:21:48 mhv3 dovecot[47532]: imap(steve)<47541><ROnt+uu9idkYgsde>: Debug: imapsieve: mailbox INBOX: Server attribute /shared/imapsieve/script not found

This is saying that imapsieve scripts are not present in your defined mail_attribute_dict.

If you're not going to use that, you might as well disable it. However, I looked at the code and this is indeed not an error and it's not causing the imapsieve processing to stop as it would without METADATA enabled.