Yes, we decided to disable XRef for all presets. But there is a good reason for that: in this way we can modify these presets on Forest updates (i.e. optimize them, update materials, etc), while keeping the compatibility with existing scenes. If not, a same scene would look different depending of the Forest version used.
Anyway you can enable the XRef option easily, editing the index.xml (in the library folder) and setting xref="1" in the first line (it is set to 0 by default).
Instead modifying the default libraries, i suggest you create a copy in the User's Libraries branch and edit it. If not, all changes will be overwritten by the Forest updates.
In Forest 4.3.6 the "Gravel" library could be xref'd just by a mistake. We disabled the XRef option in the latest releases.