Are there any non-twos-complement implementations of C?
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.
I don't have any conclusive evidence that none exist, but I've never seen one. To my knowledge, all non-twos-complement hardware was obsolete long before C was standardized.
Perhaps the best way to gather evidence would be to look for conflicting requirements and other outright bugs in the standard connected to non-twos-complement systems. If no such implementation has ever been created, it's likely there are oversights in the specification that would become apparent when somebody actually tries to make one.