Polyurethane, a pretty common plastic, which is usually in a volatile solvent but can also be water based. Its very cheap and easy to apply. It adds new material to the surface every time and is often pretty thick. Oil soaks into wood and doesn't protect the surface from mechanical damage. Its also expensive and difficult to apply. The low spots are where the decades of plastic have chipped away revealing the original wood. If you got a sample from the high spots you could count the layers like rings in the wood and figure out when the put the bench in.

Yep the only thing I can think of that would create a finish like this, and survive outside on a horizontal surface is yacht varnish

That, coupled with not sanding back before recoating would create this finish exactly. Looks to be well over a decade of recoating

Why I never!

Yeah, the lighter areas are where it has come up and been reapplied. Linseed oil gets darker with age, but I think the really dark areas are from a mold or mildew. I’ve only seen that in places with a lot of moisture year round.