No kidding: this is a problem with signed integers as well: C allows signed magnitude, ones' complement, and two’s complement representations, but no mainstream compiler supports anything other than two's complement.

> In short, the only machine the author could find using non-two’s complement are made by Unisys. Nowadays they emulate their old architecture using x86 CPUs for customers who have legacy applications which they’ve been unable to migrate.

- http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p090...