In 2021 Arch Linux 32 and FreeBSD 12.2 still works on a Pentium-class machine in text mode, specifically on a Pentium 166 MMX with 64MB of RAM.
Arch Linux 32 provides i486, i686 (Pentium Pro or better), and pentium4 builds.
FreeBSD requires i686-class machine starting with version 13.0.
On the user experience side, expect:
boot time to the text login prompt ...