Wouldn’t this be an unreliable CI though? I assume i586 and i686 cycle accurate emulators are hard to come by?

NetBSD has the same problem that the major Linux distros do, it's just expressed differently. Instead of dropping support like the Linux distros do, they will keep cross-compiling for old and obscure platforms even if nobody cares enough about them to test them. Then major breakages will start to appear that make the ports unusable (crash on boot, no video, no keyboard input, etc) and they go unnoticed for years because these ports have no actual real-world users. The only benefit I can think of for the project being set up this way is that it makes some nostalgic Gen X'ers happy when they pull up the NetBSD site and go "oh I could run a supported OS on my 68k Mac/Next Cube/Windows CE handheld/whatever, that's neat" and then they go about the rest of their day without actually doing that.