I don't think there is a clean alternative. The underlying API should still exist, but if OI10 doesn't provide a wrapper, then you might try prototyping the DLL functions or write a DLL wrapper. Are you simply interested in the functionality of MAPIAddress, or are you looking to support other MAPI related functionality as well. I guess it would be helpful to know the end goal here. It might help to identify other solutions.
Web search reveals that Microsoft Graph REST API is to be used. I would then assume that the mapixxxx stuff was pulled from OI10 as it is not really supported or encouraged by MSoft.
Graph is definitely the forward looking solution, but this isn't the "substitute" you originally asked about. This is why I suggested you provide the end goal or big picture. MAPI interacts with the Outlook profile on the desktop. Graph API interacts with M365. If you were looking for an interactive UI, you would need to build this yourself in OI and talk to the Graph API in the background.
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I would then assume that the mapixxxx stuff was pulled from OI10 as it is not really supported or encouraged by MSoft.