It is a completely ridiculous idea. You can't value "unrealized gains" without using a third-party agency to come up with a number (typically through 409a valuation). And even if the third-party agency comes up with such a number, there is no way to have liquid cash available to pay the tax. To give you an example, say you are a Founder with a Startup that received investments from investors, through various rounds of funding, and the Startup is now valued at $1 Billion. Assume also that you sit on 5 million shares, with 51% equity post all the dilution. You will have to pay tax on $510 million. This $510 million is "unrealized gain". It is an "estimate" of what you would receive if the company was hypothetically acquired for that amount by a bigger company on that particular day of valuation. Assuming 25% tax that would be $127.5 million. Where will you come up with that money? There is no secondary market where you can use your shares to raise that money. You will probably have to take a loan from banks (if they have that sort of liquid cash available for ALL unicorn startup founders/centamillionaires/actual billionaires) and that too with exorbitant interest. Why would anyone want to go through all that hassle? The other option is for you to sell some of the shares to raise money to pay tax. But that is self-defeating because you are devaluing your net worth by the same amount.
It is the most ridiculous idea ever.
EDIT:
> are doing to avoid paying tax including taking out loans against their own share portfolios
How is that tax avoidance? You do realize that when they pay the exorbitant interest on the loan, they are paying tax on the interest right? That is typically higher than if they just sold the shares and paid capital gains tax directly. Because here they are paying interest + tax.
High net worth individuals take out loans by risking their shares. Those shares are marked as lein. In other words, those shares get locked with the lender (bank in this case) and in case of the Founder not being able to repay due to bankruptcy, the lender can liquidate the assets (shares) and not be required to get the best value for it in the market.
This is not tax avoidance by any means. This is Capitalism 101: putting YOUR capital to use the way you see fit and taking personal risk along the way.