In 6.1 we changed oplog format: SERVER-65909

In 7.1 we removed support for the old oplog format from change streams: SERVER-66138

In a very rare case that:

1. A cluster is in the middle of upgrade from 7.0 to 8.0 where shard servers are on 8.0, but routers still on 7.0.
2. A change stream is reading old oplog entries from 6.0.
3. The router would include old oplog entries in the change stream filter.
4. The shard server will fail the query because it won't be able to parse those oplog entries.