Most distributions already don't support a 486 anymore. For example, Debian dropped support for Pentium with Stretch (2017). And support for 486 was dropped in Jessie (2015). 386 support was dropped with Sarge (2005).
As far as i know slackware will not boot on a 486 since some time (14.0 ?) because the kernel or glibc is compiled using instructions for later processors.