It appears it’s already been merged into the mainline and we already have the commit. After looking at the code, I figured out how to use it…
Instead of opening a new browser tab and finding the old closed thread to quote — like this one for example — Archived: the OG noob question thread! 😀 — that doesn’t work because the thread is locked, disallowing replies, which disallows quoting as well.
But if you simply search for the thread from the top bar (don’t paste a URL into the browser) in the same browser tab as the reply you are typing then you can quote the closed thread that way.
Thanks @wandagoner for finding that commit