64-bit Carbon (as a port of 32-bit Carbon, which itself was an aid for porting apps from classic Mac OS to OS X) was originally loudly announced and then relatively quietly killed[1]. Not clear if any code was ever actually written, but given the announcement was at a keynote I expect that somebody, somewhere, at least judged it feasible.

[1] https://www.macrumors.com/2007/06/13/leopard-drops-carbon-64...