Unisys 1100/2200 legacy systems use 1's complement arithmetic, and this continues in the newer Dorado series.
Rather than there "perhaps" being some still in use, they are being actively developed and have scored some 7-figure sales in recent years.
Increasingly, the microcode is now emulated using commodity hardware rather than the expensive ECL (Emitter-Coupled Logic) that older processors used, but the bangs-per-buck hasn't made it to the top of the range yet, and big banks etc are still using the big iron.
E.g. Lloyds Banking Group's core banking system runs on a 32-processor cluster of Dorado processors, and Nationwide Building Society also did a tech refresh in recent years.
The Met police use this tech for their CAD (Computer Aided Despatch) system too.
Other big users include the New York state welfare system, Nike, Subaru, NATO....