Up to a September 2018, the add-on was marked as Data Center (DC) compatible as it's just a custom field and doesn't have extra database tables or mechanism that could impact the high availability architecture. However, Atlassian rolled out a new DC program which requires a more stringent validation of the add-on.
We haven't yet applied to the new program to get the DC badge. We plan to do it within the next year. Atlassian allows you to keep using an add-on on its Server licensing model for the next year. In September 2019, DC Jira will require you to purchase DC version of the add-on by which point we should have the DC badgeChecklist for Jira is Data Center compatible since version 4.1.0.