Yes, it is opt-in, and many are unaware of its presence mostly at they simply use GETWORK and often forgo GETBLOCKTEMPLATE altogether. When their operations get to a sizable point, and the financial incentives are present to protect themselves, they will opt-in. Think of it like an in-PoW difficulty bomb of sorts.
Currently most miners make mask as all 0, which makes it impossible to prevent "block-witholding attack" from working.
Yes, it is opt-in, and many are unaware of its presence mostly at they simply use GETWORK and often forgo GETBLOCKTEMPLATE altogether. When their operations get to a sizable point, and the financial incentives are present to protect themselves, they will opt-in. Think of it like an in-PoW difficulty bomb of sorts.