I am unable to disable this on Wikinews. And, I also think why to provide an opt-out option (which doesn't work on Wikinews)? Better provide an opt-in option.
Topic on Extension talk:RevisionSlider
Hi there! The RevisionSlider is currently a beta feature on Wikinews. Click on the 'Beta' link at the top right of the page, scroll down to RevisionSlider and untick the box before saving your preferences. If this doesn't work then please post back here!
I found it. And it is working! Thank you for telling me how to do it.