@Cho, MinWoong | Jay | MPD Thank you for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
I have tested this by deploying functions app's on different plan (like consumption, elastic premium and also dedicated app service plan) in all those I don't see this clone settings from
option while creating stagging or deployment slot.
Based on the observation I can confirm that while creating the slot in function app you won't have the option clone setting from
to clone the application setting from one slot to another.
I have checked with internal team on this, and they confirmed that this feature (cloning app settings in function app) is not supported.
We see there is already a feedback item has been raised on this feature I would suggest you to upvote and make the comment on it. All of the feedback, you share in these forums will be monitored and reviewed by the Microsoft engineering teams responsible for building.
The documentation you are following is talking about creating deployment slot in Azure App service and here is the documentation for adding a slot in Function App.
Feel free to reach back to me if you have any questions on this.