I wonder what "it" in "As a writer on collecting has put it" refers to in this context
Collected objects become symbolic when they lose their practical purpose. A former prisoner in a Stalinist labor camp, for example, collected keys to locks that were no longer in use. A key is no longer just a key if it belonged to the Bastille. As a writer on collecting has put it: "Often the objects collected are the cast-offs of society, overtaken by technological advance, used and disposable, outmoded, disregarded unfashionable."
I'll really appreciate it if you answer this.
"It" refer to the whatever proposition the writer is putting. Here it refers back to the whole first part of the paragraph.
"As a writer on collecting has described this phenomenon: ..."
"As a writer on collecting has phrased these facts: ... "
"As a writer on collecting has put this proposition about keys: ..."
"As a writer on collecting has put this proposition about items no longer being used becoming items of collection: ..."
That's how I would explain it.