wayward - there are worse things to do with a legend than include a symbol not used on the map - principally getting the description of symbol wrong. Missing a symbol that is on the map from the legend (especially if its not one of the common ones) is also a bad error.
Also don't - as I once did for an offset litho map - use blue for the legend test - in fact any other colour than black is just not the most legible solution across the O population as a whole.
While ISSOM out of bounds / not-to-be-crossed gets most of the limelight, I do find myself sometimes on an ISOM map wondering whether e.g. a given piece of open land is in or out of bounds or a river is to be regarded as uncrossable (when in reality it is a paddle). The legend is then not always helpful - but can still be better than nothing. And as for the double-tick 'uncrossable' fences which range from simple post-and-wire fences not to be crossed in a country park to structures which might challenge the PoWs in the Great Escape. Always frustrating to run around, only to discover later that others have crossed - and that it was legal so to do.