Unassigned by IANA, with no service that commonly claims it.
IANA lists 8982–8988 as Unassigned, so 8987 has no registration and no protocol you can assume from the number alone. It is close enough to 8983 — Apache Solr's default — that it turns up on the same hosts as an extra or second-instance port, but that is a local choice, not a convention. Look at the process, not the number.
Expose with care
Unregistered does not mean harmless; whatever bound it may be a search index or admin endpoint that expected to be private.
$ sudo lsof -nP -iTCP:8987 -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.