Deployer's own contrib directory isn't in the default paths for RecipeLoader::loadRecipe().
Workaround: (eg. where deployer docs use require 'contrib/slack.php';, we would use the following)
\putenv('DEPLOY_RECIPE_PATHS=' . \Composer\InstalledVersions::getInstallPath('deployer/deployer') . '/contrib');
RecipeLoader::get()->loadRecipe('slack');
(PR coming!)