How did it work back when it was working?
What’s left might be a seamark:type=landmark + seamark:landmark:category=obelisk? Not that I really understand the seamark tagging scheme.
Throwing a disused: or abandoned: prefix on a man_made=beacon tag seems like it would be reasonable to me (depending on whether it’s actually missing anything from when it worked). There are currently 11 disused:man_made=beacon and 25 abandoned:man_made=beacon according to taginfo.