This tag indicates a place does not have an entry on OpenStreetMap. Sometimes this is because the place no longer exists (e.g., a demolished building).
https://github.com/davepinch/pinchy.cc/blob/master/content/topics/internet/sites/openstreetmap/no-openstreetmap-element.md
And although my own satisfaction with my work has led me to present here a draft of it, I do not by any means therefore recommend to every one else to make a similar attempt.