There are lots of such implementations (virtual whiteboard and/or pen and paper).

Search for: "collaborating writing tool", "virtual whiteboard" etc.

The latter are usually part of teleconference tools. Here's from an open source one: https://www.youtube.com/watch?v=4MNqiMEblgo

None of the ones I've ever seen or used comes close to even approximating the in-person whiteboard experience.

Not even sure what that means.

They let you write stuff (even with a pen, on a touch capable display) and have others see what you write (and even write/erase themselves).

What else is missing?

Standing in person before an actual whiteboard? Smelling the marker's ink? Maybe with the advent of cheap VR and smell-o-vision we'll get that too.

The writing part isn't really the main benefit of whiteboarding. Standing there, pointing at things and chatting about it is a really big part. Then there's the physical size. Quite often I find it much nicer to stand in front of a board and draw on a large scale. The current attempts at remote whiteboards are really not suitable replacements.

VR would a giant help as body language, gesturing and physical size all become much easier to see. Of course, that would require quite a leap forward in VR to accomplish that level of avatar.

For us left handed people it means blocking the board while I write.

You can draw things in different sizes through remote whiteboard, you can add helpful icons and your voice can get louder or quieter for effect.

Righities block the board, too, depending on the side they're on.

And you can't effectively draw some things on a tiny monitor screen.

Two things stick out as most problematic: "whiteboard" real estate and latency.

That has been my experience as well.