To be clear, you still still be able to run 32 bit binaries on a 64 bit kernel. So, there will be no more kernel support for Pentium (in 5 years or more) but you will still run i686 userland code. I do not see this changing for a very long time.

There were still new 486 compatible chips being released up until about 2010.

https://en.m.wikipedia.org/wiki/Vortex86

They released a 486sx (no FPU) in 2007.

You're probably not using anything that old for serious work outside controlling industrial machines or something but even then you cam either air gap it or use a firewall

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.