If anyone is still interested in turning layers on and off using url parameters and you're using the WAB developer edition, I've modified the url parameter handler module so you can toggle layers and sub layers on and off. I needed this functionality in an app I built and couldn't find a solution anywhere else. I posted about how to use it at https://ryandavison.net/toggle-layer-visibility-using-url-parameters-in-web-app-builder-developer-edition/ . There's a link at the bottom of the post to the updated module on Github.
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