After not having received any reports of problems and having completed some additional testing of the spine-unity runtime (including the URP and Timeline extension packages) for Unity 2019.4, we can state that Unity 2019.4 is safe to use now. So it will be advisable to use Unity 2019.4 instead of 2019.3 from now on.
In case you should find any newly introduced problems, please let us know, we are happy to fix them.