I see in the LUIS documentation, the Bing spell check capability is no longer available in the V3 LUIS API. here
But then how it is works now in composer, I mean how can I use spell check capability in bot framework composer now ?
From your link : https://learn.microsoft.com/en-us/azure/cognitive-services/luis/luis-tutorial-bing-spellcheck
The first line says "The V3 prediction API now supports the Bing Spellcheck API." So it does support it.
In Composer you can use a direct call to the API with a Send HTTP action. see the following: