D’oh! I knew I should not use the iD editor… Thanks for checking!

memorial:subject:wikidata was the first thing proposed by iD, so I used it without checking; I don’t care either way (and I’d think I wasn’t replacing subject:wikidata with memorial:subject:wikidata; at least that wasn’t my intention, anyway). (But note that the wiki page is proposed for deletion only to move the real documentation page there; unfortunately, the wiki apparently has no active admins?)

The plaque… I have no idea what happened, I probably clicked somewhere in the iD editor… it definitely was not my intention. Errrm… now I tried it and can see what happened: If you have a node selected and want to select a different node, you need to click the X in the left pane, write/paste the node ID, and click the “node” button. If, instead, you click the < in the left pane, write/paste the node ID, and click the “node” button… you’ll remove all categorization from the selected node, changing it into generic “node”. UX fail, I’d say. (But obviously my fault as well.) I reverted it.

I don’t think a natural=tree can have a “subject” (what does that mean, anyway); similarly for tourism=attraction. But moving the wikidata to subject:wikidata was definitely an option. Keeping wikidata was not. :-)