Depending on if removing collision is done with a paint layer or a sculpting tool you should have the answer to the question of if it is possible or not to do it at runtime.
Either way, given how poorly the engine runs while using landscape I don’t really see any reason to delve into any of this.
Use something other than landscape.
Mesh can be cut up at runtime via c++ and the desired result is fairly trivial to achieve.
The cut out of just about any geometry shape is possible. Complex ones too. Unlike on landscape.