A combination of jungle and mountain dead-zones can make supply routes slightly more difficult to maintain. I’ve also added the
inhospitable trait to swamp, rough_desert, pine_forest, and the (Chinese) wall. Adding this even to forests would certainly destroy many scenarios like Huertgenwald and Ardennenoffensive, so I think my choice of the most disrupting terrain types is the tolerable limit.
In Blissful & Goodtime and Operation Cannibal you can see how this creates pretty mazes. The usual supply-bummers are the combinations of roads on jungle terrain, although many original maps I checked still offered a clear path to the important objectives and supplies.

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- OperationCannibal.jpg (337.4 KiB) Viewed 3794 times
It’s slightly risky to generally use such feature for vanilla maps if you don’t know for sure what silly things the AI will do during a particular scenario.
In the example below, I have set three defensive positions with Defend Hex on the flag with distance 0. Sadly, most other inf units still danced around the flag stupidity, but at least one unit was ordered to occupy each defined position, especially the one to keep the supply route open. Without defensive positions, the AI would utterly fail in keeping the supply route to the left flag open.

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