If some guild stops playing on some server, monument will decay, buildings will decay, but terraformed land will stay forever. After some time this unnatural-looking terrain will be all over the place on the server.
My suggestion:
After some time (maybe in-game month) terraformed terrain outside of claim will become more natural.
1) There's a height differense on which dirt starts to slide down (2 meters for soil, 5 for snow, etc.). Temporary reduce this value by 2 for terraformed tiles. Dirt will slide down, forming more natural landscape. Then make this dirt look like not terraformed land (with grass, bushes, etc.).
Or
2) An easier way to implement, just reset terraforming outside of claim.
This will add more realism and if some guils leave the server, there will be fresh terrain to settle for new players.