The announcement reply in the (from original developers rejected) Feature Request issue: https://github.com/FreeTubeApp/FreeTube/issues/4447#issuecomment-3697918113

And the repository: https://github.com/D3SOX/FreeTube

And for Arch users in the AUR: https://aur.archlinux.org/packages/freetube-git-d3sox

There is a new fork of FreeTube that implements a (at least by me) long wished feature: Save video playback speed setting for each channel separately. He plans to do a Flatpak version maybe and its already in the AUR.

I personally have trust issues to run unknown forks and rely on them, for security and other reasons. So for now I won't use it unfortunately. But maybe someone else is interested.