It doesn't require a nightly build, it's available in the official Swift 6 builds too (e.g. Xcode 16 betas).
However, FullTypedThrows isn't ready yet. It frequently causes compiler hangs, for example.
See the earlier thread, Where is FullTypedThrows?.
Note in particular @hborla's comment that it's not really meant to be used yet, as it's known to be both functionally incomplete and buggy (my elaboration, not hers, but I believe that's accurate).