In short, it was an unforced error by the Biden administration to include something in its FY24 and FY25 budgets that had almost no chance of becoming law, for the sake of campaign rhetoric about making billionaires "pay their fair share."

The Biden proposal, which you can find on page 83 of this document, doesn't cover all unrealized capital gains. Most startups founders and employees would be unaffected.
First, it only applies to those with $100 million in wealth (defined as assets minus liabilities). Second, even for that group, it only would apply if 80% or more of the person's wealth comes from liquid assets (i.e., not startup stock).
If an individual is considered illiquid — e.g., a startup founder with $100 million in wealth, but $90 million of that in their company's stock — they could defer the unrealized tax payments until exit (and then it's only up to a cumulative 10% of unrealized gains).