This feature is still experimental and only visible if MuseScore is started using the '-e' option, see Command line options
For the Open-Goldberg project the ability to create different versions of the score was needed. One version is the "Urtext", a score which is close to the original version of Bach. A second version could be a Czerny variation, which in addition to the Urtext, contains fingerings.
The layer feature allows to create different versions out of one score file. If you want to add fingerings, you have to create a layer and tag the fingering elements with the layer name.
First, create layers:
Second, create a score variant:
Add the visible layer to the score variant, then select all fingerings and tag them with the name of the score variant: