UnrecognizedIFRFlag (error code 480) currently declares its UnrecognizedIFRFlagInfo extra info as extraIsOptional: true (see src/mongo/base/error_codes.yml, added in SERVER-130240 / PR #58271). This is needed during the multiversion window: a node that emits UnrecognizedIFRFlag without the extra info (e.g. an older/intermediate binary that had bare code 480) would otherwise have its error transformed into an 'Error parsing extra info' failure on a newer receiver, because mustHaveExtraInfo would reject the missing extra.

Once all supported/wire-compatible versions are guaranteed to send the structured UnrecognizedIFRFlagInfo, drop extraIsOptional so the extra becomes required (mirrors the StaleEpochInfo pattern in error_codes.yml).