It's not the case of AMD RX580. The card might not hold well OC frequencies or was re-flashed with messy memory timings, but not castrated 
There are "limited" versions of RX560, with 896 stream processors instead of 1024 (the cheapest RX560 from Powercolor, many RX560 sold in China are called RX560D - they also have 896 processors, just like the early RX460).