To my knowledge (and my limited experience with the 1 Add-In I created for ArcPro), you will need to recompile all of your ArcPro add-ins each time a new version of ArcPro is released. Hopefully this is just part of the 'growing pains' of the application and will be resolved going forward.
My guess is that the API is constantly changing and that is why it is required?? I don't remember needing to that with custom .NET coding in ArcObjects.
There is no need to recompile your add-in each time a new version of ArcGIS Pro is released. The following ProConcept explains the topic of add-in versioning in detail: https://github.com/Esri/arcgis-pro-sdk/wiki/ProConcepts-Advanced-Topics#add-in-versioning In essence there is a difference between major version releases (like 1.x, 2.x, or 3.x) and minor version releases (like 2.1, 2.2, and 2.3): only major version upgrades need to be recompiled whereas minor version releases are forward compatible, as shown in this quote from the above ProConcept:
Add-ins and configurations are forwards compatible across all minor versions of ArcGIS Pro. Add-ins and configurations are not forward compatible across major versions (eg 2.x, 3.x, etc).
You are correct in your guess that the ArcGIS Pro API is continuously changing, however, to maintain forward compatibility between minor versions features are only added to the API with minor version revisions.
Thanks for clarifying. I'm in the process of converting ArcObjects Add-Ins to ArcPro. We have about 20 Add-Ins that will need to be converted, so at least I'm clear on what to do moving forward.