In the past, SBE used to support positional and elemMatch projections. Here is the original ticket where SBE support for these features was implemented: https://jira.mongodb.org/browse/SERVER-52914

However, because of SERVER-66814, we had to disable SBE support for positional and elemMatch projections. Then, as part of de-stage-ifying "sbe_stage_builder_projection.cpp", we removed the disabled SBE implementations for these features.

Re-implementing SBE support for positional and elemMatch projections will require a new approach that works with traverseF().