Creating Task User Event Based Rollups
In v1.6.17, Task/User/Event and other objects that weren't previously supported via Custom Metadata was added to the Rollup CMDT records.
To configure these rollups, use the "(Text)" based override fields on Rollup Custom Metadata records:
Note that there are validation rules in place to prevent rollups from being improperly set up:
As these images show off, it's perfectly possible to mix both the dropdown-based selections - which have the benefit of showing up in places like "Where is this used?" - with the text-based fields. In general, the only time the text-based fields should be used to configure objects and fields is if the object you're using as the rollup child/parent does not show up in the dropdowns.