This post provides information about the user account systemd-bus-proxy.
The user account systemd-bus-proxy was used in the past by systemd-bus-proxyd service.
systemd-bus-proxy:x:999:997:systemd Bus Proxy:/:/sbin/nologin
Starting systemd-219-32, bus-proxyd was removed. Therefore, it is no longer in use in later systemd versions.
* Mon Feb 20 2017 Lukas Nykryn
- 219-32 ... - remove bus-proxyd (#1317518)
NOTE: In case that the system still have the user account, it is not harmful to leave in place.