rails--rails/activemodel/CHANGELOG.md

• Change validates_acceptance_of to accept true by default.

  The default for validates_acceptance_of is now "1" and true. In the past, only "1" was the default and you were required to add accept: true.

• Remove deprecated ActiveModel::Dirty#reset_#{attribute} and ActiveModel::Dirty#reset_changes.

  Rafael Mendonça França

• Change the way in which callback chains can be halted.

  The preferred method to halt a callback chain from now on is to explicitly throw(:abort). In the past, returning false in an ActiveModel or ActiveModel::Validations before_ callback had the side effect of halting the callback chain. This is not recommended anymore and, depending on the value of the config.active_support.halt_callback_chains_on_return_false option, will either not work at all or display a deprecation warning.

Please check 4-2-stable for previous changes.