The configuration option
sender_adress_hiding_activated = true
used to be in
plugins/com.openexchange.custom.parallels/register.js
was converted to a UI property configured in
/opt/open-xchange/etc/groupware/settings/parallels_gui.properties
defaulting to true:

# disable/enable default sender address hiding for POA setups
ui/parallels/sender_address_hiding_activated=true

The original setting was lost on every upgrade and therefore there is no automatic migration of the current setting but the default true is applied on upgrades.