HN talked about this deprecation last year, where someone pointed out that apps have been able to sign their precompiled binaries with the appropriate sandbox entitlements (no permission required from Apple!).
> UNIX hackers tend to be attracted to sandbox-exec because it looks simple even though it's not, and because doing it Apple's way requires learning a lot of Apple specific tech. Whereas the SBPL is deceptively UNIXy and simple looking.
https://news.ycombinator.com/item?id=44287740
I suspect OpenBSD has to decide whether that is of interest to them or not, if they haven’t already. Perhaps they’re unaware? Seems unlikely, but who knows with weird Apple stuff.