To add to what Dan has mentioned, yes incremental updates similar to ArcGIS Pro 2.5.0 to ArcGIS Pro 2.5.1 (not out yet). To my knowledge, Esri has not done a Compare Doc of earlier releases to new releases. With web apps for ArcGIS Online/ArcGIS Enterprise (i.e. Insights for ArcGIS, WebApp Builder and so on), the public facing side of these apps are the current iterations of the app vs. keeping older iterations on the public facing side of ArcGIS Online/ArcGIS Enterprise.
Addendum to WAB releases - one CAN download older versions of WAB here Didn't know about this site!
Looking to the future, Esri has released ArcGIS Experience Builder - a new way to envision 2D and 3D web apps. Check it out.
ArcGIS Pro 2.6? From the Dev Team - "ArcGIS Pro 2.6 Alpha 4 is now live on the Early Adopter Community." Are you a member of the Early Adopter Community program?