removeShard now clears CSS on all nodes (invalidate-all oplog entries). Under 8.0 FCV that cleanup did not exist, so dedicated ↔ embedded transitions (and stalled legacy refreshes) can leave stale UNTRACKED / UNOWNED / zero-chunk CSS. Clearing those leftovers in a backwards-compatible way on 8.0 is hard because nodes diverge and we cannot use the primary's nss set.

We currently paper over that on FCV upgrade in fixPotentiallyStaleShardingStatesAfterUpgrade by clearing no-chunks CSS (see SERVER-131968).

Instead, we should evaluate if it's better to make addShard always join with an empty CSS map on every replica-set node under both 8.0 and 9.0 FCV. That mirrors removeShard and lets us remove the FCV OpObserver no-chunks clear.