Databases running in compatibility range, upgrade recommended (SharePoint Server)
APPLIES TO: 
2013 2016 2019 Subscription Edition 
SharePoint in Microsoft 365
Rule Name: Databases running in compatibility range, upgrade recommended.
Summary: The databases have versions that are earlier than the current SharePoint software, but are within the backward-compatible range. To achieve optimal results, upgrade the databases.
Cause: Versions of databases are earlier than the current SharePoint software.
Resolution: Upgrade content databases to SharePoint 2016.
See the following resource for information about how to upgrade databases: