Try this plugin[1] which will unsign Xcode and allows older plugins to work.
I also encourage you to file a radar [2] to let Apple know you're not pleased about them killing a thriving plugin ecosystem. Yes, I know there's a new plugin API but it's so crippled that 90% of the plugins on Alcatraz couldn't possibly move to it. If enough of us complain maybe they will listen.
Haven't tested it yet with Xcode 8 but I fear it won't work.
[1] - https://github.com/kolinkrewinkel/Polychromatic