Currently RSSyl sends "libfeed 0.1" as HTTP user agent. This is a quite unpopular UA which makes user fingerprinting easier. It would be better to use something more popular: https://techblog.willshouse.com/2012/01/03/most-common-user-agents/ Ideally the user should be given a preference where he can enter (say) the 3 UA strings and Claws can use them randomly for each separate HTTP connection.
Seems to be this line: $ grep -r "libfeed 0.1" . ./src/plugins/rssyl/libfeed/feed.c: curl_easy_setopt(eh, CURLOPT_USERAGENT, "libfeed 0.1");