That's never been true. The rule is you cannot knowingly separate from your bag. Even internationally, if UA forgets to load your bag, it'll go out on the next flight.

As an aside, I had a similar situation years ago. I was flying IAH-LAX-SAN during xmas and IAH had massive delays/cancellations/pandemonium. Managed to snag a seat (literally) on IAH-SAN just after midnight (was rebooked after boarding, just handed my BP to the GA) and was elated to be onboard in a middle E- seat. At least until I realized my house keys were in my checked bag! My roommate was out of town and it was impossible to break in. By some miracle, my bag was actually on my flight. Never made that mistake again!