Debian's i386 Builds Now Require 686-Class CPUs
The Debian i386 architecture builds now require an i686 class processor for Debian testing (affecting Debian Stretch) and future builds. Support for 586 class and 586/686 class processors has been dropped, similar to the 486 CPUs being dropped previously. This i686 CPU requirement means the end of the line for hardware like the AMD K5 and K6, Intel Pentium / Pentium MMX, and VIA C3 Ezra hardware.
Debian unstable with Linux 4.3 already made a change to require 686 class processors while now GCC for i386 has been upgraded to target 686 class hardware, which will result in crashes if running on older hardware. Should you still be running any 586 hardware, you can find out more via this mailing list post.