And I really did not find the OpenWrt 18.06 'php7-mod-mcrypt' package (2019-03-11).
See also: OpenWrt Packages
I was interested in uncovering where it went and found this in the OpenWrt 18.06 php7 package changelogs for the php7.2.0 commit:
Also drop mcrypt module as it's deprecated.
So @Adrian-Aguirre I suppose you will have to review why it was deprecated and what the current best practises are around using the mcrypt functionality.
Deprecated features in PHP 7.1.x
Migrating from PHP 7.1.x to PHP 7.2.x -> Other changes