For the big companies it's pretty easy. They're largely RSU based. Shares are vested/released. Many companies allow full autosale. Easy. Even in the case of selling enough shares to cover withholding your still left with something very liquid.
Options in public companies are in basically the same boat.
The tricky one is early stages startups.
If you have options that have an exercise price at or over market value then you can take a Rule 83(b) election to defer your entire tax liability til exercise.
If the exercise price is below market then that would mean paying tax on the entire grant even though you may never receive it. Easier option for founders than employees.
Either way it doesn't cover grants along the way.
I do think it's a reasonable complaint to get taxed on something you can't liquidate.
So two things jump out at me:
1. Issuing RSUs in a non-public company send like a bad idea. Does anyone actually do this?
2. The vast majority of tax revenue would come from FAMGA shares which are already taxed so what's actually going to be gained by this? Or is executive compensation (ie ISOs) able to get favorable treatment already? This reason sooner makes it seem like a bad idea.
EDIT: an answer to my own question (emphasis added):
https://www.recode.net/2017/11/12/16640530/uber-peace-deal-r...
> Under terms of the deal agreed on, those eligible employees with stock options are capped at selling half their holdings (and those with restricted stock units cannot sell in this round).
So apparently some Uber employees have RSUs and Uber obiously public or liquid.