> You’ll find that in nearly every US jurisdiction assessed value is a percentage of market value, often a very small one.

That's definitely not the case in my municipality (North-East USA).

Tax assessments are as close to market value as you can get, considering that they are only being appraised every 5 years (I think, it may be 3 years). So there's obviously some lag if the market booms or busts. But overall it's pretty damn close to market prices and averages out over the long term.