I considered submitting a function that I deem missing to scikit-learn repo, but as of June 28th 2019 there are over 600 pending Pull Requests: https://github.com/scikit-learn/scikit-learn/pulls.
Can you explain the situation with maintainers? I mean, reviewing a pull-request is not that big of a deal compared to writing commits and also this normally has higher priority than writing code, so why I see new commits in the project, but PRs are still not reviewed?
By the way, I also found a scikit-learn-contrib project that seems to be contain many additional functions: https://github.com/scikit-learn-contrib.