Thanks for everyone’s comments – this has been shared with the WFDF Rules of Ultimate Sub-Committee.
One of the issues in mandating a ground tap is that would create an avenue for play to stop when a player fails to ground tap, which is not ideal.
I am very open to the idea of suggesting a ground tap is a good way to indicate when the thrower has established a pivot point, likely in the official annotations, but don’t think it should be mandatory.