I don't see anything particularly unique about that WoW editor, as pretty much every terrain editor used to look like that. Nowadays, the focus is more on procedural generation for higher realism and fidelity, but I'm sure there are still plenty of tools and/or plugins for hand painting them.
Some links:
- https://www.ambiera.com/irredit/
- https://godotengine.org/asset-library/asset/3134
- https://github.com/SonyWWS/LevelEditor (good luck getting it to work, but very interesting)
There are also a ton of different ways to do it in blender. You can set it up yourself, or find one of many plugins that can streamline the process.
A convoluted, impractical, legally dubious, but maximally cool alternative is to hijack a commercial game's level editor. I know at least Far Cry 2 has a level editor with a terrain system. If you can reverse engineer the level format, you can write a script that'll convert them to whatever format you need... Just make sure you don't keep the meshes and textures from the game.
Not until Kent pkills his AI waifu