Thanks stragu, that's a good idea to do the query to see how people tag these time capsules. 

I did a query just now on https://overpass-turbo.eu/ to search for all nodes with "Time Capsule" in the name to see how they were tagged. (I'll post the query code at the end.) These are the result I came up with after a bit of data massaging:

historic=memorial: 28
name=Time Capsule: 25
historic=monument: 11
memorial=plaque: 8
historic=time_capsule: 7
historic=yes: 3
tourism=attraction: 3
leisure=park: 2
historic=time capsule: 2
tourism=artwork: 2

The winner is historic=memorial. To me however a time capsule is not really a memorial. But I could live with it. Maybe it memorializes the age? It gets rendered as a gravestone though, which is unfortunate for playground.

I do like historic=time_capsule, but one problem is that it's not a standard tag, so it doesn't get rendered on the map. What is the point of having something on the map if people can't see it?

I still don't mind tourism=artwork, but I think none of the choices are great.

Regarding the artist name, I found that here: https://davesculptures.com.au/playspaces

And here: https://www.facebook.com/groups/KeepWestEndWeird/posts/10161448904765299/

I'll email him once we sort out the tagging to get all the details.

----------------
This is the overpass query, made with help from the wizard:

[out:json][timeout:100];
// gather results
(
  // query
  node["name"~".*time capsule.*",i];
);
// print results
out body;
>;
out skel qt;