Any free software OS can meet his requirements as they all are transparent to anyone with the time to learn, just like the older bikes. The problem with modern bikes is that all the advanced tech is proprietary and you have no way of understanding it or tinkering with it.
IMO you can take any modern distro and strip it down to something understandable. It just takes some time to learn how to strip it down and how what's left works (and this is ongoing, as things are always changing). I'm not saying it's trivial, but neither is learning how to rebuild a motorcycle.
All systemd related processes (dbus, systemd-*, init, etc) running on my system are using < 25Mb of memory. So that is a bit excessive, even if exaggerating for effect.
I don't normally get into these, but this is blatantly false, I'm running systemd on multiple older/smaller computers like an OG Raspberry Pi and an old netbook, and I don't even notice its memory consumption.
I still have my 75 MHz Pentium 12MB EDO RAM laptop running Windows 95 and holding my old BBS sitting in my drawer. Battery is shot but plug it in and it still works.
IMO you can take any modern distro and strip it down to something understandable. It just takes some time to learn how to strip it down and how what's left works (and this is ongoing, as things are always changing). I'm not saying it's trivial, but neither is learning how to rebuild a motorcycle.