I've been experiencing it either way on my phone this week. As I'm quite judicious about using "private" sessions for everything I suspect its been under a/b test.
Yeah, this feature being part of an A/B test makes sense.
Hopefully the results of said test show enough of a drop in traffic/conversions for the variation with the 'login requirement' that they drop the idea altogether.
There doesn't seem to be any downside to old/ud - it retains users that would otherwise leave whilst being enough of a pain that those who the wouldn't leave just login.
As you say, its not present on ud. or old.