Most of the SDK docs are online.
Not all of us is upgrading immediately.
I could not find anyway to get to the docs for 2.1 or 2.0
From here ArcGIS Pro SDK | ArcGIS Desktop
From there... Home · Esri/arcgis-pro-sdk Wiki · GitHub
scroll to the bottom
Release ArcGIS Pro 2.0 SDK for .NET · Esri/arcgis-pro-sdk · GitHub
Release ArcGIS Pro 2.1 SDK for .NET · Esri/arcgis-pro-sdk · GitHub
and of course, if you select under Resources.... ArcGIS Pro 2.2 API Reference Guide
you end up at the version 2.2 reference guide
Assuming this is what you mean
If you need to get the wiki pages in the arcgis-pro-sdk repo for the previous versions -
They are available as "branches" in the wiki section of the repo.
Suggest you give an upvote and use case to ArcGIS Pro Help should start showing version numbers (e.g. 1.0-2.1.2) . ESRI is under the impression that you upgraded to 2.2 the day it was released, and will continue to upgrade to the latest version 3-4 times per year with every SP and version change (What happened to the online 2.1.x Help? ).
Retrieving data ...