It might be handy to have some of the escapes that work in $'string' quoting to also work in prompts especially now with the ${parameter@P} transformation.
Specifically the hex, unicode and control ones: \xHH, \uHHHH, \UHHHHHHHH and \cx.
I presume that the dollar-single-quote escapes should not be touched since they are "specified by the ANSI C standard". Also, they needn't be since we have the @P transformation.
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