This feature might not be available in 3rd-party clients, like polls and cards.
Worst case, take a photo of your screen with another phone. The harder it is to scrape the content, the more vulnerable the content posted there will be, which will make it that much more desirable a target to scrape.
If it's not on the website it'll be DOA.
Facebook usage is > 80% mobile, and they often don't bother adding features to the website. Do you have reason to believe that Twitter is different?
What features aren't available on the website that are available in the mobile app?
I've seen some news stuff that isn't I suppose, but not much actual functionality.
It is possible to detect when that screenshot is subsequently shared within the platform. Twitter could prevent blocked users from sharing screenshots of tweets by the people who have blocked them, they just choose not to because eNgAgEmEnT.