Splay doesn’t exist on any current controllers, there are three levels of tracking in OpenVR, Estimated (most controllers), Partial (Index with curl), and Full (none currently, but things like gloves and/or leap motion or AR pose tracking could provide).
Index controllers don’t have a splay element, so the splay values will always be empty.
The hand action has a var “SkeletalTrackingLevel” that you can access to read out, I will need to make a function that retrieves that eventually as well. I’ll note that the module is fully compatible with the official Valve plugin, when that one is installed mine turns off the input handling and just acts as a layer on top of it to allow for the gesture / replication / full body posing.