Hi 👋

I believe that code like this which "prematurely pessimizes" common operations should not be listed as an example.

The reason that was added (seven years ago) was to replicate the behavior of Lodash which is pessimistic about the existence of the input. Replicating Lodash should be the purpose of this repository, of course, so that flexibility is a requirement.
Now, whether that is performant is important. But, Lodash is not training-wheels for JS in the same way that jQuery was... it should be pretty obvious to most developers how to access the last element of an array. The point of its inclusion in Lodash was for brevity and convenience in a time before default arguments and optional-chaining existed as language features (and had majority browser support).

IMO, these examples could be replaced with the actual implementation which is relatively brief:
https://github.com/lodash/lodash/blob/ddfd9b11a0126db2302cb70ec9973b66baec0975/lodash.js#L7595-L7612

function last(array) {
  var length = array == null ? 0 : array.length;
  return length ? array[length - 1] : undefined;
}