(because "save as" isn't available on "modern" websites)
I think this is also more of a mobile issue with apps more than just websites. Like, hence why those screenshots are so often from phones. Generally one's that are more obviously from a computer are social media posts, presumedly to facilitate sharing on other platforms easier.
Because on desktop, even if a website doesn't allow it, using the dev inspector usually allows you too. I wonder if it would be possible to create an extension/userscript that automates that on hostile websites.