As of my knowledge cutoff date in September 2021, there have been no official announcements from Microsoft regarding support for connecting to offline cube files using ADOMD.NET in .NET Core or .NET 5+.
One of the reasons for this is that the transition from .NET Framework to .NET Core and the newer .NET versions has involved a significant refactoring of libraries and dependencies, with an emphasis on cross-platform compatibility and performance improvements. Some older technologies and features, such as support for offline cube files, might not be prioritized for newer .NET versions due to low demand or because they are considered outdated.
It's worth noting that the open-source community has created a project called ADOMD.NET Core, which aims to provide an ADOMD.NET client library for .NET Core. However, this project is not officially supported by Microsoft, and it may not provide full compatibility with offline cube files. You can find the ADOMD.NET Core project on GitHub here: https://github.com/NetCoreApps/AdomdClientNetCore
In conclusion, there are no guarantees that Microsoft will add support for offline cube files in .NET 7 or future .NET versions. If your application relies heavily on this feature, you might need to continue using .NET Framework 4.5 or explore alternative solutions, such as migrating your data to a modern data storage and analysis platform.