This file is indexed.

/usr/share/doc/plum/copyright is in plum 2.33.1-10.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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This package was debianized by GOTO Masanori <gotom@debian.or.jp> on
Tue,  2 Nov 1999 13:12:23 +0900.

It was downloaded from ftp://ftp.madoka.org/pub/plum/

Upstream Author: Yoshinori Hasegawa <hasegawa@madoka.org>

Copyright:

Copyright (c)1997-1999 Yoshinori Hasegawa <hasegawa@madoka.org>
The full text of the GNU General Public License is available on Debian
systems in /usr/share/common-licenses/GPL.

Note that, there is no license description in the source code.
The upstream maintainer mailed me as follows (EUC-JP encoding):

Copyright¤Ë¤Ä¤¤¤Æ¤Ï»ä¤¬¤Ï¤Ã¤­¤ê¤È°Õ»×ɽ¼¨¤·¤Æ¤¤¤Ê¤¤¤¿¤á¤Ë
³ÆÊýÌ̤Ǥ¤¤í¤¤¤í¤È¸æÌÂÏǤò¤ª¤«¤±¤·¤Æ¤¤¤ë¤è¤¦¤Ç
ÂçÊÑ¿½¤·Ìõ¤Ê¤¯»×¤Ã¤Æ¤¤¤ë¤Î¤Ç¤¹¤¬¡¢¤³¤ì°Ê¹ß¡¢plum¤ÎCopyright
¤Ï
GPL2¤Ë½¾¤¦¤è¤¦¤Ë¤·¤Æ²¼¤µ¤¤¡£

(He declared that the license of the plum package follows GPL2.)