Hi Jason,
H2 is not packaged by default, but will come back in if you use certain plugins, like NetCDF/GRIB, a flavor of GeoFence. It’s also used as cheap replacement for postgis in image mosaic, WFS, and other security tests.
As part of the GeoServer 3 effort we are eliminating it, fully, from the codebase, this discussion is about eliminating one of the usage points, other PRs are already coming in to remove it from GWC (optional but still possible) and as a downloadable datastore plugin.
Cheers
Andrea