ISO 19111 states clearly that "Nesting of CCRSs shall not be permitted - the individual single systems shall be aggregated together". Despite this clear specification, GeoAPI allows aggregation of CompoundCRS. The rational was that databases like EPSG define three-dimensional CompoundCRS with (x, y, z) axes and valuable additional informations like the domain of validity and the identifier. A user may want to add a t axis. If nesting CCRS are not permitted, then that user must break the 3D CCRS in its SingleCRS components and create a new 4D CRS from the components. In this process, the above-cited informations associated to the 3D CCRS are lost or not applicable to the 4D CCRS.
We need to decide to either forbive nested CCRS as specified by ISO 19111, or document this departure from the standard. This depends on whatever the information associated to the 3D CCRS are considered valuable enough for introducing a departure, or not.