Currently, the ArcGIS Pro Help only shows the data for the most recent version of ArcGIS Pro. Yet, there have been breaking changes between different versions of Pro.
This issue affects especially arcpy development. Some arcpy classes have had breaking changes, that unfortunately cannot be looked up in the Help, since the Help only shows the most recent version.
For those developers seeking to maintain backward compatibility, it would be really helpful if the Pro Help showed version numbers like for ArcMap, and allowed selection of a previous version: