the file 'function.json' has specified an invalid trigger of type 'entityTrigger' and direction 'in'. Currently, only httpTriggers are supported.

That's an error produced by the GitHub action when trying to deploy a Static Web App with some Durable Entities in its /api folder.
More precisely:

> Function Runtime Information. OS: Linux, Functions Runtime: v3, Node version: 12.X
> Finished building function app with Oryx
> Found and validated 'routes.json' within build artifacts folder.
> Error in processing api build artifacts: the file 'AppointmentEntity/function.json' has specified an invalid trigger of type 'entityTrigger' and direction 'in'. Currently, only httpTriggers are supported. Visit https://github.com/Azure/azure-functions-host/wiki/function.json for more information.
>
> For further information, please visit the Azure Static Web Apps documentation at https://docs.microsoft.com/en-us/azure/static-web-apps/
> If you believe this behavior is unexpected, please raise a Github issue at https://github.com/azure/static-web-apps/issues/
> Exiting
>

Obviously everything works perfectly on the devbox.
Why is it that "Currently, only httpTriggers are supported"?
Which component introduces this strange limitation and why?
With "only httpTriggers are supported" there isn't much we can do on the backend side apart from "hello world" scenarios!