Just looking at the language myself, but it seems that it treats out-of-bounds array access as a non-recoverable bug and panics [1, 2], whilst map access returns Option [3]. Exceptions are a language construct only to enable Java compatibility and not recommended otherwise [4], but that's not to say you couldn't implement your own try/catch using the effect system. `r` is a region variable as the sibling comment says.
[1] https://doc.flix.dev/chains-and-vectors.html#vectors
[2] https://flix.dev/principles/ See also "Bugs are not recoverable errors"