This bug (invalid method call) hasn't been caught by unit tests because Vagrant::Plugin::V2::Config catches all invalid/bad configuration calls and save them for generating error messages during the "validate" stage. This way, the `ask_sudo_pass=(value)` method was not interrupted and the `@ask_become_pass` attribute was (surprisingly) correctly set (allowing the related unit tests to pass). In order to avoid similar problem to happen again, the deprecation message output is now fully verified.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.