The Linux kernel just deleted IA64, so that support won't last for long, unless Gentoo keeps ancient kernel versions? Debian still has unofficial ia64 and powerpc ports too.
It doesn't, but the kernel also isn't package managed, just the source. You can keep around the ebuilds for older kernels in an overlay, or someone might maintain an enthusiast overlay just for IA64 kernel support.
What do you mean "it doesn't"? I'm maintaining Debian Ports and I build installer images for all old and obscure targets. We even have support for SuperH which Gentoo dropped at some point.