Not sure. I know one corner of my parent's land in Montana has a physical marker. Maybe because it's a joint boundary between state, federal and private land. It is also really hard to find, as it's in the bottom of a ravine in heavy brush and it's a ground level bronze (I think) spike like an oversized nail hammered into the ground. You really have to know where the boundary is within a few feet already to have even the slightest hope of finding it.

I found it once as a teen, but 2 decades later, I'd be hard pressed to find it.

That might also be a USGS reference marker, used for topographic surveys.

Did it look like one of these? https://en.m.wikipedia.org/wiki/Survey_marker

Could be. On further reflection, I also recall small plastic spikes that were significantly smaller (about an inch in diameter) on the private boundary. Only found one or two of those.

Nope. They're still used at the very least in the US and Germany.

There are different versions of it - most common are probably a metal nail (if you have stone or something else to anchor it in), a metal pipe (if dirt is all you've got) or a massive, pain-in-the ass to lug around stone. (My dad's a surveyor, and in my youth I was cheap labor. I still hate boundary marker stones :)

It's AIUI not _necessary_ to have them, but if you have a property dispute, they come in handy.