Opensource OTRS plugins developed by AllWorldIT are available for download below.
This plugin extends OTRS to provide a customer-specific means to add notes which are displayed for each ticket associated with the customer's ID.
Download: https://download.allworldit.org/awit-otrs-customernotes
Development: https://gitlab.devlabs.linuxassist.net/awit-otrs/awit-otrs-customernotes/