Imapsieve plugin?

[Bruce]

Debian stretch.
Dovecot 2.2.27

I was working with a document that references the imapsieve plugin.  It 
references the /etc/dovecot/conf.d/90-imapsieve.conf file.

The only reference I've found for it is 
https://wiki2.dovecot.org/Pigeonhole/Sieve/Plugins/IMAPSieve#Configuration.

I've got dovecot-managedsieve and dovecot-sieve already installed.  Am I 
missing something obvious here?

Bruce M.