NetBSD does not support 386 anymore either and I think NetBSD requires an FPU. With its x87 emulator, it may be that Linux has been more hardware friendly in this case.

Here is a Linux for 486 that runs in 8 MB (current kernel, same userland as Alpine Linux): https://github.com/marmolak/gray486linux/commits/master/

There will still be LTS kernels supporting 486 in Linux until 2030 or later. The oldest kernel currently still getting updates at kernel.org is a version from 2019.

Outside the official kernel project, distros like Ubuntu and RHEL offer 10 years of support. So, they will be dropping security updates for these kernels for even longer.