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You'd need to word it carefully so that people who took out mortgages on houses weren't hit with capital-gains liabilities when the house value went up.
I suspect it would be difficult to enforce because the borrow-don't-sell model is specifically there as a work around for paying taxes. If you make it illegal, you're incentivising the next tax dodge. My guess is that it would be something like the shenanigans Musk is doing, where one company he owns buys other things with shares, but there are infinitely more complex systems that someone who can pay enough accountants could come up with.
Simply taxing assets above a threshold total when they go up in value, whether you realise it or not, also has the issue that it's hard to accurately value private companies. Their valuation is based on the value of the last investment round and the amount it sold for, but it would be relatively easy to set up a bunch of companies that, on paper, consistently lost value and so offset net-worth gains. I wouldn't want to pass such a law without having a team of accountants and lawyers look at it for loopholes before I did.