Discussion/more info here: https://forum.unity.com/threads/severe-taa-ghosting-and-blurriness-because-terrain-detail-mesh-trees-do-not-write-motion-vectors.1182244/

Hi,
It seems you misunderstood the bug report, it's not about that higher quality TAA should give better results. TAA documentation states: " The default settings for higher presets are not guaranteed to produce better results than lower presets. The result depends on the content in your scene."

The issue is much bigger than that, it's that grass/foliage, when rendered through instancing, it loses motion vectors which TAA needs to do a good job.
Hence in the project you can see a specific grass rendered through terrain 'detail object' looks blurry in comparasion to the other grass which is rendered normally as a mesh placed on the level. Because the latter has motion vectors, and the former (the one rendered through unity's terrain detail object) loses it's motion vectors.

And you have to use instancing with grass, otherwise the high density of grass will kill your performance cause of drawcalls and overhead. You have to use instancing/indirect instancing, otherwise you can't have a forest, or any large amounts of grass. which is why terrain uses instancing to render grass.