A framework is a library that is so pervasive that you cannot reasonably do so - your shim would just be a copy of the framework's API, and it would be exceptionally difficult to take a different framework and make it fit the shim.

E.g. if you need to work with time zones, you can easily write out the function signatures you need, then look up any number of date-time libraries and make any one of them fit your shim.

But if you need to 'manage data flow between backend and UI', and try to write out the function signatures you need, you're either going to be copying a particular framework's API or you're going to have a hell of a time implementing the same signatures for two different frameworks if one is one-way and the other is two-way.

The old archetype of a framework might be one which expects to control an overall event loop for your application.

In practice many things we would still recognise as frameworks allows you to undo that, but I still think it's a useful rule of thumb in that even many frameworks which technically allows you to remain in control of the overall flow tend to assume the framework will mostly dictate structure and flow.