It felt like “bit rot” to you because that’s how you defined “rot”, but it’s not how real rot works, which has nothing to do with changing.

It’s something that just breaks down without any changes in use case. If a wooden bridge rots away, nobody means that they were unable to add extra lanes. They mean it ceased to do what it previously did.

“Code calcification” is likely a better approximation of what you’re trying to convey.

“Bit rot” would be better fitting to actual storage loss, but this industry chose poorly.