It was removed earlier - either DP5 or DP6. Now the only way to get to the setting is through System Preferences > Accessibility > Display and tweak the settings for transparency there.
Maybe they disabled the fancy effects on certain older hardware because of performance issues. Maybe there was something wrong they had to temporarily disable functionality or simply a bug on certain hardware.
I'm confused. I got a MBP 2011 myself, and the option to enable translucency is there in Accessibility. That's the way it has been set up since 10.10 DP1. Can you please clarify?