Unassigned at IANA, with no widely used default service.
IANA lists 8185 through 8189 as one Unassigned block, so nothing is registered on 8189. There is no common product default here; a listener is a local choice, most often an application or web service on an arbitrary free port. Fingerprint it rather than inferring a protocol from the number.
Expose with care
Unregistered and unidentified is not the same as harmless — find out what answers before you decide.
$ sudo lsof -nP -iTCP:8189 -sTCP:LISTENSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.