As far I know "system folders" like Draft, Sent and Trash are created by the IMAP application (webmail) not Dovecot but is there a way to suppress that process and let Dovecot do that up front of a webmail?
I have found some configuration options in
/etc/dovecot/conf.d/mailboxes.conf which allows to auto create and auto subscribe those "special use" mailboxes but I'm not sure if this is what I'm looking for. If so are those "system folders" are going to be created before user login to a webmail or during login process, which will be the same as to use a webamil functionality to create those folders.
My Dovecot setup uses mbox format and SQL for users storage.