They subject all of their dark patterns to multivariate testing. I suspect this is to elicit feedback like yours, and suppress the ability to respond to it, as they have never in years promoted any of it to general availability.

I’ve been in tests where the entire site is gated, demanding I download the app. I’ve been in tests where SFW content is marked as NSFW, demanding I log in. Etc.

Why do these companies think they can "test" users anyway? Do we look like guinea pigs to them?

They don’t view you as a user... most companies who are ad supported fundamentally view everything as an impression/page load which is simply a container for ad placements. Click through, conversion rates and revenue per thousand impressions are the only metrics that matter to the health of the business.

Everything else is secondary and most product managers don't view a/b tests as anything other than experiments on ways to increase those stats.

I just see two buttons: One that says "Log in", another that says "I'm not over 18"

This is correct. You have to swap to old.reddit.com to get in without a login (for now).

In the US, the seem to no longer require logging in. If they geo-locate your IP to anywhere else, they show "Log In" and "I'm not over 18" buttons.

its completely depends

I still routinely run into it on my phone where I cannot view the site at all unless I download the reddit app, especially if its a NSFW subreddit