I actually just spoke to Delta about this because my flights were a codeshare on Virgin but booked through them. My flight from LHR directly to IAD was not showing up in any search engines (Delta, Virgin, Google). The agent said it was still in her system but when she tried to book, it would not let her. She said everything was zeroed out even the flight was officially still operating. I noticed that the only flights I could book back to DC were ones through Atlanta or New York. Oddly enough, my direct flight from Seattle on the way over (mid September) is still showing up in the Virgin and Delta booking engine so that one is still valid.