MLT 6.18.9 was published 2 days ago and, besides bug fixes and stability, implements a few features that are much wanted in the upcoming 19.12 Kdenlive release, like audio mixer. Without MLT 6.18, some Kdenlive features will not work, and it would degrade user experience.
PS: There is no "release-team" member available in reviewers, as we are told to do in the Release Schedule