"context rot" is more aligned with what they are describing, though that's not a metaphor. I can see bitrot being a bad fit, as it's decay of the actual thing rather than the surrounding context.

Except that implies the context has rotted, when in fact the context is fresher than the system within it. While the context may have degraded in some way, the reality is that it's changed according to forces that have acted on it since the code within it was written and deployed. What's rotted is the ability of the system to meaningfully and correctly interact with the context.