we use Github actions to deploy our function apps, we first deploy it to a slot then swap it.
But I noticed this, should I add the AzureWebJobsStorage app setting to Azure Function Slots as well or ignore this warning? If so, is there a way to avoid this warning?
Yes, it's required.
It's used for storing configuration (bindings, triggers, etc.) and logs related to the function app. As the errors states, it is mandatory.
It's not just some magic floating thing, the function app requires its own storage for its own setup and configuration.
https://learn.microsoft.com/en-us/azure/azure-functions/functions-app-settings#azurewebjobsstorage