Alright, your observation is indeed correct. Soil now uses a different theme support key and an array for the options:
You have to replace the existing Soil-related lines in Sage 9.x (and currently 9.1, I plan to fix that):
/**
* Enable features from Soil when plugin is activated
* for Soil 3.x
* @link https://roots.io/plugins/soil/
*/
add_theme_support('soil-clean-up');
add_theme_support('soil-jquery-cdn');
add_theme_support('soil-nav-walker');
add_theme_support('soil-nice-search');
add_theme_support('soil-relative-urls');
/**
* Enable features from Soil when plugin is activated
* for Soil 4.x
* @link https://roots.io/plugins/soil/
*/
add_theme_support('soil', [
'clean-up',
'disable-rest-api',
'disable-asset-versioning',
'disable-trackbacks',
// 'google-analytics' => 'UA-XXXXX-Y', // @TODO: Add Google Analytics site key
'js-to-footer',
'nav-walker',
'nice-search',
'relative-urls',
]);