Only in Europe. Its still approved by the USDA.

Source(failed petition to remove it): https://www.ams.usda.gov/sites/default/files/media/Rotenone%...

What does "organic" mean if a specific substance can become "not organic" without changing?

You have a point. I think "not organic" in this context is shorthand for "not allowed in organic agriculture." Scanning the reg, the USDA's list is titled "Prohibited Natural Substances."

If you think the term "organic" is silly: in German the word that's used means "biological".