My source is simply reading history :) It has been reported that during time of Omar ibn Abdul-Aziz, there were no poor people left to take Zakah, since everyone paid their dues.
The difference between Zakah and wealth tax as proposed in the links you mentioned, is that the former applies to almost everyone (except the poor/needy obviously) in lieu of income and sales tax, not just to people with > $500k (or whatever) in their accounts. It may not be as straight forward to calculate compared to income tax, but the goal here is to solve a problem, not to just implement a mediocre solution that has been shown not to work.
I came across this article that gives a high level summary[0].
Can you source this? Because I understand the exact opposite to be true.
https://www.npr.org/sections/money/2019/02/26/698057356/if-a...
https://www.npr.org/2019/03/01/699261950/why-a-wealth-tax-di...
https://www.businessinsider.com/4-european-countries-wealth-...
https://taxfoundation.org/elizabeth-warren-wealth-tax-oecd/
tldr: European countries dumped them because it is hard to calculate value of assets, especially for the ultra rich.