Hello pandj, 

There was a decision to limit the number of internal libraries/functions that are returned in the expression builder. If you type the call in by hand, it should still work, but finding the function has become a bit more challenging if you're looking for the ones that product management decided not to include. The following support article doesn't talk about @ObtainValuePair(), but I think it's explaination/alternate way to get the full list back should be applicable.

https://community.pega.com/support/support-articles/business-calendar-functions-are-not-shown-expression-builder

Thanks,

Mike