Confirmed there are still some broken links in the monitoring-plugins from impish:

triage-impish+21.10:~/pkg/MonitoringPlugins/monitoring-plugins-gu/plugins$ egrep -sr https?:// *  | grep -v www.gnu.org | grep -v example.com | grep -v 192.168.
check_curl.c:* http://curl.haxx.se
check_curl.c:                     * http://support.microsoft.com/kb/318380/en-us */
check_dbi.c:	printf (" %s\n", _("driver. See its documentation at http://libdbi-drivers.sourceforge.net/"));
check_dns.c:  /* https://www.cryptobells.com/verifying-ips-in-a-subnet-in-php/ */
check_dns.c:  /* http://computer-programming-forum.com/47-c-language/1376ffb92a12c471.htm */
check_game.c:  printf (" %s\n", _("http://www.activesw.com/people/steve/qstat.html before you can use this plugin."));
check_http.c:    /* @20100414, public[at]frank4dd.com, http://www.frank4dd.com/howto  */
check_ide_smart.c:	printf (_("This plugin checks a local hard drive with the (Linux specific) SMART interface [http://smartlinux.sourceforge.net/smart/index.php]."));
check_mrtg.c:  printf ("   %s\n", "http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/mrtg.html");
check_mrtgtraf.c:  printf (" %s\n", "  http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/mrtg.html");
check_nt.c:* (http://nsclient.ready2run.nl/)
check_nt.c:	printf ("   %s\n", "(http://nsclient.ready2run.nl).");
check_nwstat.c:  printf (" %s\n", _("  (available from http://www.engr.wisc.edu/~drews/mrtg/)"));
check_overcr.c:  printf ("%s\n", _("Over-CR can be downloaded from http://www.molitor.org/overcr"));
check_ping.c:  printf ("%s\n", _("the contrib area of the downloads section at http://www.nagios.org/"));
check_snmp.c:	printf ("%s\n", _("http://net-snmp.sourceforge.net before you can use this plugin."));
check_tcp.c:		SEND = "<stream:stream to=\'host\' xmlns=\'jabber:client\' xmlns:stream=\'http://etherx.jabber.org/streams\'>\n";
check_ups.c:  printf (" %s\n", _("http://www.networkupstools.org"));
picohttpparser/picohttpparser.c:             * http://www.mozilla.org/security/announce/2006/mfsa2006-33.html */
t/check_http.t:        $res = NPTest->testCmd( "./$plugin -I $host_tcp_proxy -p $port_tcp_proxy -u http://$host_tcp_http -e 200,301,302");
tests/check_snmp.t:Technical Support: http://www.cisco.com/techsupport
tests/check_snmp.t:Technical Support: http://www.cisco.com/techsupport
tests/check_snmp_agent.pl:Technical Support: http://www.cisco.com/techsupport
utils.h:    https://www.monitoring-plugins.org/doc/extra-opts.html\n\
utils.h: https://www.monitoring-plugins.org/doc/guidelines.html#THRESHOLDFORMAT\n\

---

1. https://www.cryptobells.com/verifying-ips-in-a-subnet-in-php/
   - Link works, but there's no content...

2. http://www.activesw.com/people/steve/qstat.html
   - Resolves to generic tax software review article unrelated to qstat.
   - User-visible string

3. http://www.frank4dd.com/howto
   - 404 & bad cert

4. http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/mrtg.html
   - Cannot connect to server
   - User-visible string

5. http://nsclient.ready2run.nl
   - Domain name up for sale
   - User-visible string

6. http://www.engr.wisc.edu/~drews/mrtg/
   - 404
   - User-visible string

7. http://etherx.jabber.org/streams
   - 404
   - Used in a testCmd() call

If better links aren't known, then at least it would make sense to move the bad links out of user-visible strings into internal code comments (they may have some value as historical reference).  And I agree with Nish that this is probably more something for upstream than for us to carry as a packaging patch.