Hi EM From the FAQ above:
Q: I am currently using the Store for private line-of-business apps built by a software vendor or in-house developer. Will those continue to work in the new solution?
A: Line-of-business apps will need to be migrated to your local private app repository before the retirement of the Microsoft Store for Business in Q1 2023. We will provide more details on how to do this migration for you and your ISV in the near future. Apps that have already been assigned and deployed to endpoints will continue to work after the retirement of the Store for Business as long as they are not removed from the device.
...and since this is the next logical question:
Q: How will app updates work in this new solution?
A: MSIX apps from the Microsoft Store will still be automatically updated. Other installer types or MSIX apps distributed directly from Microsoft Endpoint Manager can be updated by Microsoft Endpoint Manager.