A tax on unrealized capital gains is in effect a tax on assets. And for that, there’s at least one historical example. Before they had oil wealth, Norway had a tax scheme that taxed the assets of their wealthiest citizens, not just their income. IIRC, some of their wealthiest citizens ended up paying tax that worked out to more than 100% of their income. The sky did not fall.

But whether Harris would be able to get this plan through Congress and then have it survive judicial review is an open question.