This has been a question hovering for a long time.
Question is simple:
I noticed VS warned me of outdated RhinoCommon nuget package, as I’m sill supporting Rhino V7 and installed the v7 version of the nuget package.
If I upgrade to the latest v8.17, do I still have backward compatibility for my plugin to support Rhino v7? Or does it mean that only Rhino V8.17+ will be supported?
No
This.
For both Rhino 7 and Rhino 8 use the corresponding nuget package of the minimum service release you want to support with your plug-in. So you still can (and should) target multiple major versions. Just make sure you have your build system set up to create builds for both versions.
But updating to latest major nuget package is necessary only if you need access to API that does not yet exist in the version you are using now.