The web is steadily moving to HTTPS-only:
so having only HTTPS download urls is normal and actually inevitable novadays. Drop the annoying warning, as there's not much a maintainer can or should do about it.
If HTTPS actually poses any problems to users behind a proxy, and we want to help them, we need to provide HTTP mirror for all distfiles in a centralized way. We actually do with distcache.freebsd.org.