Yep however that only exposes a value of "last time the user registered/verified their account via phone number activation" and "last day the app connected to the signal servers".

There isn't really anything you can do with that information. The first value is already accessible via other methods (since the phone companies carry those records and will comply with warrants). And for pretty much anyone with signal installed that second value is going to essentially always be the day the search occurred.

And like another user mentioned, the most recent of those warrants is from the day before they moved to username based identification so it is unclear whether the same amount of data is still extractable.