It's not relevant yet, but the C23 standard requires 2's-complement arithmetic.
In document N2912 (available from the committee website
https://open-std.org/jtc1/sc22/wg14/ under 'documents'), which is the June 2022 draft of the standard, section 6.2.6.2 Integer types says:
NOTE 2 The sign representation defined in this document is called two’s complement. Previous revisions of this document
additionally allowed other sign representations.
So, in a decade or two, we will be able to ignore systems which do not use 2's-complement arithmetic. If you were ruthless, you could probably ignore 1's-complement and sign-magnitude systems now — this is very largely recognizing the status quo. There probably are a few mainframe antiques around with the alternatives, but not many.