Despite the theoretical portability of implementation code used in functions, the surrounding features and tooling make it relatively difficult to deploy the application with another vendor.
For Azure Functions, specifically, Microsoft has recently released the Azure Functions Runtime, which allows you to run functions on your own server. With Azure Functions Runtime, you can run functions on-premises or even in a different public cloud, which allows you to avoid vendor lock-in. For more information, visit https://docs.microsoft.com/en-us/azure/azure-functions/functions-runtime-overview.
