Not sure if it can be classified as technical debt but I worked at a place that had a very clever and profusely productive architect. As the company grew, some perks available to developers got cut to the point he decided to move on.

From that point on, the architecture froze. It still was possible to continue creating new features on top of it, but I am sure that architect would be able to provide both guidance and new solutions I feel may be needed. No one else picked up his tasks or views, nor was the architect position filled up.