```ruby
# frozen_string_literal: true

module DeviseHelper
  # Retain this method for backwards compatibility, deprecated in favour of modifying the
  # devise/shared/error_messages partial
  def devise_error_messages!
    return "" if resource.errors.empty?

    render "devise/shared/error_messages", resource: resource
  end
end
```

Add a deprecation warn for `DeviseHelper.devise_error_messages!`
user
```ruby
# frozen_string_literal: true

module DeviseHelper
  # Retain this method for backwards compatibility, deprecated in favour of modifying the
  # devise/shared/error_messages partial
  def devise_error_messages!
    ActiveSupport::Deprecation.warn <<-DEPRECATION.strip_heredoc
      [Devise] `DeviseHelper.devise_error_messages!`
      is deprecated and it will be removed in the next major version.
      To customize the errors styles please run `rails g devise:views` and modify the
      `devise/shared/error_messages` partial.
    DEPRECATION

    return "" if resource.errors.empty?

    render "devise/shared/error_messages", resource: resource
  end
end
```
assistant