in 2e I had a pack of wights lead by a vampire take a party of high teens (the straight wizard was 15 or 16) down to all single digits... it was pretty bad. 1 took out a pad, wrote some notes, then ripped her fighter/thief character sheet and took out a blank one cause it was easier to start over with a clean sheet then erase and back up everything.While I've never whacked a group from 18th to 7th (hell, I've never had a group get anywhere near 18th in the first place!) I too have removed numerous levels from characters over time.
That campaign the Wizard died around level 12 after that. However that Fighter/thief and 2 other characters got to use high level campaigns at one point...