If there are no sidewalks mapped anywhere, it means the note is pointless. Everybody can see there are no sidewalks mapped. In most places we do not have the sidewalks mapped, it is “normal”.
I think a note for missing sidewalks would only make sense if the general picture is that of complete sidewalks, but at one spot it is not mapped despite there is one. Or somewhere there is a tag “sidewalk=no” and instead there is a sidewalk.
I agree with @SomeoneElse, if there are missing features, rather than adding notes, add the features.
IMHO not, every information is optional, there is no strict requirement to add anything. To put this into perspective, in Italy there are currently 1 155 472 highway=residential streets, 60800 of them have a sidewalk tag (5%), there are also 110000 footway=sidewalk tags for separately mapped sidewalks (regardless of highway type)