Here for example my Mythic Hounds of Sargeras kill before prepatch: https://www.warcraftlogs.com/reports/2Y6BtPVzpXQKM7kT/#fight=last&type=damage-done and I can’t kill them anymore in prepatch on same character…
I don’t have logs for other mythic antorus bosses I soloed before prepatch (Aggramar, Argus the Unmaker) because I decided to log it last moment on last week before prepatch and i’ve already killed them earlier same week and unfortunately didn’t think about enabling logging…