Technically, you just add a subdivison surface modifer to the end of your stack and set that to adaptive.
If that is a good idea, though, depends on your scene.
The adaptive subdivsions benefit from having a base with very little polygon count as it’ll only subdivide parts of the mesh until the pixel/verts ratio is met.
The subdivision surface modifier just flat out increases your polycount. And adaptive will not de-subdivide stuff.
So, just using a plane and shader displacement will result in high detail landscapes with relatively low resource needs.
There are still reasons using the displacement modifier or a combiation of both.
I my case it’s because “microdisplacement” is confused when the UVs arent connected and because I don’t want the fine details being pushed in the same vector as my overall structure.
Comparison: The same object, using only adaptive subdivsions:
In the upper left part of the image you can see that the object becomes disconnected where the uvs are seperate.
And front center you can see that it just pushed everything straight up, which does not look that “organic” anymore.
I think for a landscape, if you don’t want to actually model the overall geometry, you could use both, but:
You’ll have to use different textures for your displacement modifier and your microdisplacement and you’ll have to find a balance between the settings of both subdivision modifiers.
If you need a high count for displacement, you might have to reduce the adpative subdivsions. (Or buy more RAM)