I don't know. Essentially every place I've contacted I've asked about that clause. Not one actually expected to find it, but they all listed it.

5+ years seems to me to be a strange expectation. That would mean you've had either multiple failing jobs and might be an undesirable, or you've been somewhere for 5+ years. If you've been there for 5+ years, what's motivating you to leave? Where do people expect to find these vast pools of highly-skilled jobless people who have experience with <software stack X> in <field which employs a couple thousand people nationally> within <narrow time window>? That they never find any seems to underscore how irrational the 'requirement' is in the first place, but I see it everywhere.

I remember reading a job ad back in 2003 that wanted someone with 5+ years of C# and .NET experience. I assume the position was filled before 2006. (It's possible but unlikely that they were solely looking to poach Microsoft employees.)

This happens a lot. HR ends up being in charge and comes up with requirements that don't make much of any sense.

I think it's breaks down like no experience = SDE 1

2 years = SDE 2

5+ = SDE 3, regardless of language.