Thank you @Sargon and @Harald. I was just researching Timeline today. :party:

By any chance, will there be support for 2020.1 ?

This is probably a long shot. But does Spine work with Unity Recorder? I tried, but had no luck :wounded:
https://docs.unity3d.com/Packages/com.unity.recorder@2.2/manual/index.html

We can gladly announce that Timeline support for SkeletonGraphic along with multi-track preview (multi-track scrubbing) in the Editor has finally arrived! Thanks again @Sargon for sharing your implementation for SkeltonGraphic in the meantime!

The new 3.8 UPM package is available for download here as usual:
Spine Unity Download

Hope you like it!

I have added a section to the spine-unity docs pages for further reference:
spine-unity Runtime Documentation: Timeline Extension UPM Package

Forum user Sargon has kindly shared his implementation for SkeletonGraphic on this forum thread:
Spine timeline plugin for 2019.1
Thanks again @Sargon!

The Spine timeline package has been updated, now providing the previously removed features and more.

See the documentation section here:
spine-unity Runtime Documentation: Timeline Extension UPM Package

From the changelog:

SpineAnimationStateClip now provides a Speed Multiplier, a start time offset parameter Clip In, support for blending successive animations by overlapping tracks. An additional Use Blend Duration parameter (defaults to true) allows for automatic synchronisation of MixDuration with the current overlap blend duration. An additional Spine preferences parameter Use Blend Duration has been added which can be disabled to default to the previous behaviour before this update.

You can download it here as usual:
Spine Unity Download: Download

We hope that now all previously provided Spine Timeline features have been re-activated and the additional features make it more comfortable to use: now you can finally drag your animations over each other to create transitions.

Official Timeline support for SkeletonGraphic and SkeletonMecanim is yet to come, but at least SkeletonGraphic is within reach - stay tuned for updates.