In FCV_AND_FEATURE_FLAG_README.md document the technical requirements for feasibility checks that may be added to _userCollectionsUassertsForDowngrade or _userCollectionsUassertsForUpgrade, for example:

• Feasibility checks are expected to be fast (<10s in reasonable circumstances).
• Feasibility checks should not scan large datasets (e.g. user data, chunks, etc.).

Additionally note that feasibility checks during the upgrade (i.e. removed features) need to be reviewed by Server Release.