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.