`extend`ing objects at runtime

I've always avoided extending objects at runtime, like in this example:

class Polymorphic
  def initialize(style: :foo)
    extend Something if style == :bar
    # ...
  end
end

It used to be that performance could be severely impacted but it's not clear it's still the case today (see https://github.com/rubocop-hq/rubocop/pull/8881 )
Style-wise, it seems to complicate things and make introspection / debugging / branch-covering more difficult.

Is that an accepted principle to avoid this (and if so, is it still valid)?

Review the runtime extend example and the linked RuboCop pull request #8881 to establish whether the performance concern still applies. Then determine whether the style guide should state a principle about avoiding runtime extension and what wording would resolve the open question; done means a documented, agreed-upon guideline.