Hello @JGauthier1971 !
I’ve checked Landscape deformation in UE501 and it works as intended.
Please take note that “Apply Spline to Landscape” button won’t work if “Edit Layers” option is enabled for the Landscape.
If you want to modifiy a layer of a Landscape use the Blueprint Brush instead.
To use the Blueprint brush: go to Landscape Mode → Sculpt → Blueprint and select BP_SR_LandscapeBlueprintBrush, make sure the layer you want to modify is selected and then click anywhere on the landscape to create a brush actor, then in the brush actor Details panel click “Select All Roads” and “Update Landscape” buttons.