If the site supports https://
then use that. If not, then use https://
.
https://rpc.technorati.com/rpc/ping
https://blogsearch.google.com/ping/RPC2
https://ping.feedburner.com
https://rpc.icerocket.com:10080
https://services.newsgator.com/ngws/xmlrpcping.aspx
https://api.feedster.com/ping.php
https://rpc.pingomatic.com
https://rpc.weblogs.com/RPC2
]]>
That list is for pinging services to announce your post. Since the post is public I don’t see any harm in using the http for the service that still remains. It’s not like you’re passing confidential or sensitive information and your own URLs that are sent can be https.
]]>Really?
The following is the list included in WP 4.9.8 (At first glance only two are missing from the list above: https://services.newsgator.com/ngws/xmlrpcping.aspx and https://rpc.icerocket.com:10080):
https://api.feedster.com/ping
https://api.moreover.com/ping
https://api.moreover.com/RPC2
https://blo.gs/ping.php
https://blogsearch.google.com/ping/RPC2
https://bulkfeeds.net/rpc
https://geourl.org/ping
https://ipings.com
https://ping.blo.gs/
https://ping.feedburner.com
https://ping.syndic8.com/xmlrpc.php
https://ping.weblogalot.com/rpc.php
https://rpc.blogrolling.com/pinger/
https://rpc.pingomatic.com
https://rpc.technorati.com/rpc/ping
https://rpc.twingly.com
https://rpc.weblogs.com/RPC2
https://www.blogdigger.com/RPC2
https://www.blogshares.com/rpc.php
https://www.blogsnow.com/ping
https://www.blogstreet.com/xrbin/xmlrpc.cgi
https://www.feedsubmitter.com
https://www.newsisfree.com/xmlrpctest.php
https://www.pingerati.net
https://www.pingmyblog.com
https://www.weblogalot.com/ping