This package is deprecated 
In versions 8.8 and 8.9 with dev mode enabled, the plugin will not compile when using classes from this package. You can disable it for these two versions, but ultimately in version 9.0+ they will not be available in any way.
List of deprecated packages - Include/exclude list for RFC-24 Data Center grey API removal - #5 by scott.dudley
How to turn off dev mode - Preparing for Confluence 9.0 - EAP coming soon - #82 by rr1
Cheers
Adam