2.93 is this a bug? The lighting is dead simple with a sun lamp. The object is quite large at 300 meters but applying scale makes no difference. Same result with blender 3.0 beta.
I just tested on a similar scene. I solved that problem by doing this:
1-Go in the render settings, in the shadows section and increase the “cascade size”. This will increase the resolution of sun lamp shadows.
2-Go in the lamp’s settings and increase the shadow’s “bias” until the artifacts disappear. This will slightly detach the shadows from objects and prevent the artifacts, which are caused by objects self-shadowing.
3-Now, we fix the shadows being detached by activating “contact shadows” and adjusting their distance so they are just long enough to fill the gap. Contact shadows should be kept as short as possible for your situation, because they are not fully calculated in 3D. They are very precise for small details, but if they get too long, they can become inaccurate, misunderstand the scene’s depth or ignore objects that are off screen.
The contact shadows also have a bias setting, but the default should work fine.
4-If there is still a problem, you can adjust the “max distance” on the sun lamp so the shadows are properly scaled for your scene.
5-Also, the main render samples may need to be increased so the pixelated look of the shadows has the time to become smoother (as long as you have not turned off soft shadows in the render settings).
I have tested the scene further.
The artifacts only appear when shadow cascade count is more than 1.
The other settings will change the appearance of the artifacts but not making it disappear.
I will file a bug report.