Thanks to @etaionshrd on Reddit I found this Terminal command:

    defaults write com.apple.dock slow-motion-allowed -bool true && killall Dock

If it doesn't work, try copy-pasting it. There is no auto-correct in Terminal and spelling counts. Works for me macOS 10.13.2 High Sierra.

https://www.reddit.com/r/apple/comments/6jeefp/in_the_latest_high_sierra_beta_a_small_relic_from/#t1_dmpcle3