Hey Chris,
I am in a similar situation as the OP. I have downgraded to ArcGIS Pro 2.9.11 to deploy a solution with my 10.8 enterprise. However, I am having a hard time locating the ArcGIS Solutions Deployment Tool add-in. It seems all of the original links to the tool direct to the current form of Solutions, which assumes the enterprise 11 workflow.
Do you have any idea where I can locate the installer for the add-in?