My understanding is that 64-bit time fixes are only needed for 32-bit architectures, based on Debian's notes about the time_t migration project: https://wiki.debian.org/ReleaseGoals/64bit-time. 64-bit apps already have a 64-bit time_t, at least in Debian (and I assume Ubuntu too) with their standard compiler settings. It's mostly for 32-bit ARM CPUs. 64-bit architectures still need to be tested since build/code changes can unintentionally affect them too.