"OK, I figured out what happened. It wasn't ideal in terms of commits. So, we have an old configuration system GConf, and we replaced it with our new one, GSettings, for N reasons. GSettings was missing a feature for gnome-terminal to implement, so it went without GSettings for a long time, until the 3.8 cycle, when we ported it over with a workaround. The port wasn't 100% complete, so it landed with the intention of adding feature parity, like with terminal transparency support.
However, after asking our team, we decided it wasn't worth it keeping the support code around for terminal transparency, and dropped it.
It's an unfortunate thing that happened where the reasoning wasn't relayed entirely in commit messages, and you're right -- it was dropped in a seemingly unrelated commit. But that's after intentions changed -- a broken port to new tech with the intention to fix it, then that changed, and thus it got dropped under the guise of "Remove dead code"http://www.reddit.com/r/linux/comments/1e4p53/gnome_37_removes_terminal_transparency_closing/