Are there any other regular expression variants that support that?

I didn't see any other regular expression variants of golang that support \C. I see the use of it in the syntax wiki of re2.

There are no plans, because it requires operating one byte at a time, and the regexp implementation is fundamentally rune-at-a-time.

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

Closing as infeasible.

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group