The field-based variables we used by default during 9.1.x already
were not volatile, which is the direction we want to move people.
Volatility should be achieved through libraries like
concurrent-ruby, or added to Ruby as a way to declare volatility
at class definition time (attr_accessor :foo, volatile: true).

The risk here seems small since 99% of variable accesses have
already been nonvolatile for over a year, all major concurrency
libraries are using the proper mechanisms that don't require
instance variable volatility, and we have received no bugs where
nonvolatile field-based variables were the cause.

Non-volatile instance variables