There's a way to measure "entropy" of a codebase. Take something like the binary lambda calculus or the triage calculus, convert your program (including libraries, programming language constructs, operating system) into it, and measure the size of the program in it in bits.

You can also measure the crossentropy, which is essentially the whole program entropy above minus entropy of the programming language and functions from standard libraries (i.e. abstractions that you assume are generally known). This is useful to evaluate the conformance to "standard" abstractions.

There is also a way to measure a "maximum entropy" using types, by counting number of states a data type can represent. The maximum entropy of a function is a crossentropy between inputs and outputs (treating the function like a communication channel).

The "difference" (I am not sure how to make them convertible) between "maximum entropy" and "function entropy" (size in bits) then shows how good your understanding (compared to specification expressed in type signature) of the function is.

I have been advocating for some time that we use entropy measures (and information theory) in SW engineering to do estimation of complexity (and thus time required for a change).