As we all no doubt know, the ISO C standard (and C++ as well, I think, though I'm more interested on the C side) allows three underlying representations of signed numbers:
The most recent example I can find is the UNISYS 2200 series, based on UNIVAC, with ones-complement arithmetic. The various models were produced between 1986 and 1997 but the OS was still in active development as late as 2015. They also had a C compiler, as seen here.
It seems likely that they may still be in use today.