Pretty much anything related to the work environment. This can include maintaining a safe work environment, accommodations for specific employee needs, handling employee disputes.
I don't see how this isn't still necessary.
You don't see why maintaining a work environment wouldn't be necessary if the entire company were allowed to work from home?
The work environment remains, it just shifts location.
Do you see how these problems greatly diminish if people cannot interact physically?
That's a fraction of what HR does. Compensation, benefits, hiring, wellness, training, performance reviews, etc, etc. I doubt that even 10% of what HR does is tightly related to a physical space.