Given the lack of source control, Azure backups, or retrievable storage account data, recovering the deleted function is highly unlikely.
If the function app was hosted on a Dedicated app service plan, it can be restored, as long as it was using the default App Service storage. See:
https://learn.microsoft.com/en-us/azure/app-service/app-service-undelete?source=recommendations
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Regards,
Abiola