Recompiling add-ins with ArcGIS Pro SDK for .NET version 2.8 that were made with previous versions results in target platform mismatch warnings similar to the following.
Warning: There was a mismatch between the processor architecture of the project being built "MSIL" and the processor architecture of the reference "ArcGIS.Desktop.Mapping", "AMD64". This error can also refer to other namespaces, such as ArcGIS.Desktop.Framework. This mismatch may cause runtime failures. Please consider changing the targeted processor architecture of your project through the Configuration Manager so as to align the processor architectures between your project and references, or take a dependency on references with a processor architecture that matches the targeted processor architecture of your project.
Starting at ArcGIS Pro 2.8, a number of the ArcGIS Pro extensions are built using the x64-bit platform target (rather than AnyCPU) to accommodate the latest C F# upgrade. Continuing to compile add-ins with a platform target of AnyCPU at ArcGIS Pro 2.8 and above will result in these compilation warnings.
The warnings can be ignored but, starting at ArcGIS Pro 2.8, Esri recommends changing the platform target of add-ins and configurations to x64 to remove them.
Note: This is not an issue for new add-ins made with the ArcGIS Pro SDK for version 2.8. Starting at version 2.8, the default platform target for add-ins is already set to x64.