The concept of pagebundles should not be part of public stable APIs. For this reason, the transform endpoint exposed by MW core should not support them.
However, the legacy transform endpoint exposed by the parsoid extension needs to continue to support pagebundle. Since both endpoints are backed by the same handler class, the set of valid transformations needs to be configurable, rather than being defined as a constant in ParsoidFormatHelper::VALID_TRANSFORM.
If it turns out that we need pagbundle based REST endpoints internally, separate endpoints should be exposed at paths that is inaccessible for the public. These endpoints could be backed by the same Handler class that handles HTML input and output, since a PageBundle is just HTML with extra data wrapped in JSON.