Yes, I know there should be landuse or natural, that's why at first I added natural = ground;grassland.

So maybe better tagging would be
natural = grassland with
surface = ground and
a note or check_date.

And yes, this area was used as free space during adjacent roadworks - for example for temporarily stack removed old road signs and some construction's materials.

It's possible a grass will grow again (you're right that's "private land"), but rather after longer time as it was heavily used. But it's not certain, an owner could as well sold it, I don't know yet.

So basically I wanted to add an info about current state, so it could be easier spotted to re-check later how it goes with this area.

But well, it's not that important, so can stay as grassland and if something will be changed here then we can update it later.