Yep, gone from the AGR transfer page. No announcement on either Amtrak's or United's websites.

But for whatever it's worth, I found AmEx MR points --> OP --> AGR a better deal anyway than redeeming MR points for Amtrak certificates or redeeming on UA; 25,000 points is worth the equivalent of 5 unreserved coach, 2.5 reserved coach, or 1.5 Acela business 1-region round trips (e.g. my WAS-NYP trips being around $180 usually, 25,000 points buys at least $450 worth of travel on Amtrak versus maybe $375 if I redeemed for an airline transcon).