SERVER-77280 allowed for the OR stage to have clustered collection scans as its children, and will unblock the bug described in SERVER-61259. However, the query below for a clustered collection fails: 

coll.find({ $or: [{ _id: { $lt: 1 } }, { _id: { $gt: 8 } }] }).sort({_id:1});

The generated plan is MERGE_SORT with 2 CLUSTERED_IXSCAN children. In SBE this tassert is hit, but this generated plan works in classic. Therefore, SBE should support these plans and support sort keys when building clustered collection scans.