Is it possible to deploy only one function even though if stack has multiple functions?

Unfortunately it's not possible with how cloudformation works. Can you tell me a bit more about why you want to do this?

But SLS does it, how it does it?

They deploy it manually outside of cloudformation using the aws api directly. I think we need to understand the workflows where people want to do this better so we can think about how to address them

That makes sense.

I need to onboard the customer based on their features request. Let's assume x client want a,b,c feature which present in stack and y client want a,b feature which present in the same stack. So while onboarding the customer I don't want to deploy the c feature for y client.

Are you deploying a stack per customer? You should pass in props to the stack to determine what to deploy

No. I'm not using stack per customer. I don't want to deploy a function in stack based in customer feature request. Do you've any example?

You can pass in props into the stack to determine what to deploy. I'll write an example soon, however I'm not sure how you can deploy multiple versions of your app without creating a new instance of a stack per customer