In the first half of 2025, I implemented Whittaker-Henderson smoothing in the PR ENH: signal: add Whittaker-Henderson smoothing by lorentzenchr · Pull Request #22580 · scipy/scipy · GitHub .
Discussion for inclusion already happened, positively. What is missing is a review. I am posting it here for a bit of visibility (and before I completely forget what I’ve done 6 months ago).
Interesting fact: Whittaker-Henderson smoothing might be older than any of the implemented functionalities in `scipy.signal`, yet it is still missing (in a release).