I'd seen this 4/19 date stated in a few posts with no seeming backup, so I contacted the queen of all Camaro info/my favorite Camaro salesperson, Becky, and here's what she said today:
"The 19th was the last day for unallocated sold orders - orders dealers could take without needing any Camaro allocation to place them. This week and next are the last for allocated orders. So if the dealer has Camaro allocation they can place a sold order against it."
So, posts stating no one can order a Camaro after 4/19 are not accurate. Anyone wanting to place an order should probably check with Becky/dealerships for allocation quick, and good luck!