This originally came from a search HELP ticket linked below. The customer downsized from 2 shards to 1 shard and began seeing "Invalid ShardedSearchPlanDefinition" whenever $searchMeta queries are attempted with an inner exception 'BsonParseException: unrecognized fields ["mongotQuery", "metadataMergeProtocolVersion", "limit", "sortSpec"]'. This looks a lot like https://jira.mongodb.org/browse/CLOUDP-225635 / https://jira.mongodb.org/browse/SERVER-85694 which only seemed to affect 7.2+.

Slack thread: https://mongodb.slack.com/archives/C039SULHYGH/p1711663412839869
Original search HELP ticket: https://jira.mongodb.org/browse/HELP-57729