A short while ago I posted a summary of the current state of play of Azure Functions and .NET 5. In short, to run your function in .NET 5 you need to use the new Isolated Process. It’s so new that it’s missing a lot of the Azure Functions features, e.g. several bindings and Durable Functions. So Durable Functions users are stuck on .NET Core 3.1 until .NET 6 is supported in the In-process version.
Whilst all that is still true, there is now an update from the team on where they’re intending to go in future. The In-process version will end with the .NET 6 release and development will concentrate on bringing the Isolated Process up to feature parity in time for .NET 7. Read their post here. After that they are promising to support .NET versions as and when they are released.
This is best illustrated by reposting their roadmap from that link:
The Durable Functions support in the Isolated Process is said to arrive in “2022 or possibly earlier”. I look forward to it.