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It's not authentication safeguards, it's dark UX patterns driving you to signup. Whether or not you'll see someone's profile and how much of it you'll see depends on how they fingerprint you, where you navigate from, and on the phase of the Moon. LinkedIn is notorious for that.

My memory might be misleading me here, but I think I remember years ago in one case I could see less info about a profile when logged in to my account than I could see while logged out and navigating from a Google search...

Nope. You can see public profiles without being logged in. Certainly the case here in the UK; not sure about elsewhere.

Even twitter requires you to be logged in to view "replies" on a user's profile.