With https://gerrit.wikimedia.org/r/#/c/96536/ , Echo is deployed on all 877 WMF wikis. So echowikis.dblist is unneeded (it's the same as all.dblist except for ordering). (it is no longer on all wikis).
In that commit Benny comments:
@Todo - echowikis.dblist is used by a cron to send daily/weekly digest email, with Echo enabled on all wikis, this file becomes redundant, we can set the cron to execute with foreachwiki blah.php instead of foreachwikiindblist echowikis.dblist.
This is class misc::maintenance::echo_mail_batch in puppet/manifests/misc/maintenance.pp
So first that should be changed in Puppet, and then echowikis.dblist and references to it can be removed from mediawiki-config configuration.