> LIB_DEPENDS without a WANTLIB entry associated with that port is not
> usually valid. Check "FLAVOR=upnp make port-lib-depends-check".

Thanks, here is the corrected Makefile diff.

Index: net/i2pd/Makefile
===================================================================
RCS file: /cvs/ports/net/i2pd/Makefile,v
diff -u -r1.24 Makefile
--- net/i2pd/Makefile	27 May 2024 08:13:51 -0000	1.24
+++ net/i2pd/Makefile	31 Jul 2024 14:25:02 -0000
@@ -2,7 +2,7 @@

  GH_ACCOUNT =	PurpleI2P
  GH_PROJECT =	i2pd
-GH_TAGNAME =	2.52.0
+GH_TAGNAME =	2.53.1

  CATEGORIES =	net
  HOMEPAGE =	https://i2pd.website
@@ -20,6 +20,15 @@

  # for tests
  USE_GMAKE =	Yes
+
+FLAVORS+=upnp
+FLAVOR?=
+
+.if ${FLAVOR:Mupnp}
+LIB_DEPENDS+=net/miniupnp/miniupnpc
+WANTLIB+=miniupnpc
+CONFIGURE_ARGS+=-DWITH_UPNP=ON
+.endif

  WRKSRC =	${WRKDIST}/build