It was replaced several years ago by the stable option `-C
overflow-checks`. The goal was to delete the `-Z` flag once users had
migrated [1]. Now that it's been several years, it makes sense to delete
the old flag.
See also the discussion on Zulip [2].
[1]: https://github.com/rust-lang/rust/issues/33134#issuecomment-280484097
[2]: 262497224