I don't know what website you are reading, but the Zcash website says it is 20% now but after 4 years drops to nothing, and when you account for the dropping rewards given to minors, after ten years (when mining will end) the result will be 10% went to the founders: so people saying "a 20% tax" are correct today even if the tax rate will amortize a long time from now to only be 10%.

> At first, 50 ZEC will be created every ten minutes. 80% of the newly created ZEC will go to the miners, and 20% ZEC to the founders.

https://z.cash/blog/funding.html

> Every four years, the rate of ZEC being created will halve (again, just like in Bitcoin). After the first four years the ZEC created per ten minutes will drop to 25ⓩ, but after the first four years, 100% of it goes to the miners.

> The end result (as shown in the diagram) is that there will ultimately be 21 million ⓩ, and 10% of it, or 2.1 million ⓩ, will have been initially distributed to the founders.

It's 10% of all ZEC over planned mining-distribution, but front-loaded: 20% of mining-rewards for the 1st four years, then 0% thereafter.