I am posting here because Pro is what started my desire to find this.
Where can a person get, hopefully in one place, a comprehensive list of changes to software. For example, I had been forgetting to update my Pro software and did it just recently, however, now only then did I discover that I must upgrade License Manager to continue to licensing Pro through Portal. Why a minor version change requires changing a existing, supported, server stack is an aside with strong opinions; but where could I find this information? I found it on the software requirements page for Pro 2.9 but nothing in the release's blog post, which seems mostly focused around marketing.
Is there a secret sys admin page where I can just a list, hopefully less pictures and more text, of changes from version to version? The ArcGIS Pro 2.9 Issues Addressed seems to only address bug fixes and I could not find any notice of the License Manager requirement. The Release Notes page for ArcGIS Pro 2.9 doesn't seem to address this either.
First I want to say I totally and completely understand your pain! Secondly, yes, I think the documentation is out there but you said you found the official pages of ESRI published documentation and it didn't suffice.
My only other suggestion would be to pay close attention to jumping a whole number in the upgrades... that is where some of my major issues have surfaced in the past (example: from 2.6 to 2.7). This forum amongst other forums will also give you some user based "gotcha" information about the different releases. Geonet AKA "ESRI Community", GIS Stack Exchange, and well... good ole Google! I know it not much of the answer you are seeking but I hope this helps!
Maybe we could look at License Manager perspective. Supported software products—License Manager Guide | Documentation (arcgis.com)
I would always advice to keep the License Manager updated. Most of the lower version(s) of clients (ArcGIS Desktop, ArcGIS Pro, ArcGIS Engine, ArcGIS CityEngine) are compatible with them.