Invisible objects only visible in mirror - How? (in Blender 2.8 or so)

The issue I am having here is in two parts:
I was looking for tips on how to make objects invisible and their reflections visible when rendering a scene, but those were all for older (or should I say outdated) versions of Blender, and the more recent ones were irrelevant to what I needed help with, which is this:

If I have a specific object (mesh) and I put it in front of a plane mesh that I made into a mirror, then I will see a reflection of that object.
Right? However, what should I do to make the the object, and not its reflection, invisible?

Also, if I select faces of an object, how do I hide those from view but keep their reflection?
Hint: The mesh is rigged, so if I make the selected vertices a separate object, it would affect their armature deformation.

Those are both issues regarding versions of Blender starting from 2.8 (I am currently using 2.91).

Re the first part.