First of all, read the UPGRADE guide from react-admin.
Since API Platform Admin is built on top of react-admin, almost everything in the react-admin upgrade guide applies to API Platform Admin as well.
This UPGRADE guide will only cover the specific changes for API Platform Admin.
Since the way to define custom routes has completely changed in react-admin, the way to add authentication support in API Platform Admin has also been modified.
In short, you need to use the <CustomRoutes> component inside the <HydraAdmin> or <AdminGuesser> component, with a redirect condition on its child.
The condition is taken from a state variable, and the state updater function is given to the data provider and will be used when there is an unauthorized error.
To see the full updated example, please go to the related documentation page.
Since react-admin does not use Redux anymore, it's also the case for Mercure in API Platform Admin.
Instead it uses react-query cache to update the received changes in real time.
You will not see the Redux data action when a resource is updated by Mercure anymore.