Here is the commit (and subsequent discussion) where Memoizable was deprecated: https://github.com/rails/rails/commit/36253916b0b788d6ded56669d37c96ed05c92c5c
The author advocates the @foo ||= ... approach and points to this commit as an example for migration: https://github.com/rails/rails/commit/f2c0fb32c0dce7f8da0ce446e2d2f0cba5fd44b3.
Edit:
Note that I don’t necessarily interpret this change as meaning that all instances of memoize can or should be replaced w/ this pattern. I read it as meaning that Memoizable is no longer needed/wanted in the Rails code itself. As the comments point out, Memoizable is much more than just a wrapper around @foo ||= .... If you need those features, go ahead and use Memoizable, you’ll just have to get it from somewhere other than ActiveSupport (I’m guessing someone will fork a gem version, if they haven’t already).