250.68(A) Accessibility. All mechanical elements used to terminate a grounding electrode conductor or bonding jumper to a grounding electrode shall be accessible.
Exception No. 1: An encased or buried connection to a concrete-encased, driven, or buried grounding electrode shall not be required to be accessible.
Exception No. 2: Exothermic or irreversible compression connections used at terminations, together with the mechanical means used to attach such terminations to fire-proofed structural metal whether or not the mechanical means is reversible, shall not be required to be accessible.
I have normally been required to mount a mud ring and a cover plate. This always confused me because it hardly made the connection accessible for any kind of repair. I would guess this was just for a visual inspection. Also, this was very much AHJ dependent. Even in the same jurisdiction, different inspectors wanted or did not want.