No Reddit/YouTube/Facebook/Twitter I'm not installing an app just to use your browser based service. Stop asking every time I browse your site.

Is it a mobile browser feature or something built-in to the website?

I've read Chrome used to have some setting to disable it, but of course Google removed it because users found it too useful.

No idea why the (SFW!) content I wanted to look at was considered "18+" in the first place; it was just a discussion of a company that I was curious about. I can't help but think that they're getting more and more liberal in applying this content flag, since it must drive either account creations or app installs (and often both). I even have a Reddit account, but now I just can't be bothered to follow any search results there anymore.

The web is really getting smaller and smaller by the day, with large platforms gobbling up content and engagement under the guise of being "just another website/forum/blog/..." at first, but inevitably ending behind a paywall, login-wall, or in a mandatory app install, and often even all three.