Where is "Reader Mode" icon in address bar of Safari 15 for macOS?

In Safari 14, if "Reader Mode" was available, a "page" icon appeared next to the address bar. Clicking that icon activated Reader Mode for that page.

In Safari 15, that icon is gone.

For example, see this article in Wired, Review: Apple iPad Mini (2021).

I find I can still activate Reader mode by choosing the menu item View > Show Reader. Ironically, after doing so that page icon appears in the old location, available to turn off Reader Mode. But how can I similarly turn on reader mode with a quick click of an icon?

Reader Mode does not properly recognize the article you've linked to. It would not have appeared in previous versions either.

Reader Mode icon appears within the address bar on the left side