It should be opt-in per website, per feature, because IMO it can be quite useful in some cases. Like clicking back on a slide-show bringing you to the overview page, instead of only going back one slide

> clicking back on a slide-show bringing you to the overview page

That behavior is expected in exactly one case (assuming slides, not the whole presentation, are modeled as a page each): If I navigated to that specific slide from the overview.

In any other scenario, this behavior amounts to breaking my back button, and I'll probably never visit the site again if I have that choice.

Opt in features are a great way to increase user frustration and confusion. See the whole new geolocation API they had to make for browsers since people would perma-deny it reflexively and then complain that geolocation features weren't working.

That's a good point, though I'm not familiar with the (changes to the) geolocation API you mention. Do you have any recommendations for reading up on that development?

Sure, I should have said geolocation element, since the original API still exists and is used: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...