Are there any pre-release (alpha/beta) versions of Vaadin Flow 13 available somehere?
The web site keeps changing, finding new places to hide the pre-release info.
The [old releases page]
(https://vaadin.com/releases/) no longer has all the info. It now has only a description of features of the latest release, Vaadin 12.
The [RoadMap page]
(https://vaadin.com/roadmap) says v13 is due in only 3 months, yet has no information about its development nor links to access any pre-release product.
There does not yet seem to be any pre-releases for 13. I would expect the first alpha to be released in the near future, after which it will be available at least on the GitHub page you linked, possibly also on the starter page.
Is there a feature list what will be implemented in vaadin 13?
Look at that roadmap link posted by Erik Lumme above. It shows a column of tiles for each upcoming version. Each tile seems to be a feature. I do not know if that list is exhaustive. https://github.com/orgs/vaadin/projects/1