Ice Cubes will be iOS 17+ only in a couple of weeks. Don’t worry if for some reason you can’t update your iOS version you’ll still be able to use the last available update of the app. As long as Mastodon is not making any breaking change in their API (and they most probably won’t) you’ll be fine. 
Don’t forget to update your devices to iOS 17 (if you can) on Monday! You’ll get a much faster Ice Cubes for free!