At Pro 2.5 The .NET Framework is Changed to 4.8
With the release of ArcGIS Pro 2.5, SDK developers will now be using .NET 4.8 as the minimum target framework.
For customers running previously deployed add-ins at 2.0 through 2.4, no recompilation is necessary. Pro is forwards compatible with regards to add-ins and the upgrade of .NET Framework does not change that.
However, if you recompile an add-in written on a previous version at 2.5 (or better) you will get compiler errors - even if you have not made any changes.
The assembly references for ArcGIS Pro may also show as being broken even though the paths point to the correct location or you have run the utility "Pro Fix References":
To recompile an add-in, previously created at version 2.0 – 2.4, using 2.5 (or better) you must change the Target framework in the Application properties to 4.8. You should also change the desktopVersion attribute of the Config.daml to 2.5 (to reiterate, add-ins are _forwards_ compatible not _backwards_ compatible)
Complete instructions can be found here: How To: Convert a version 2.0 to 2.4 ArcGIS Pro SDK add-in solution to Pro 2.5 and later versions