2019-08-10 20:05:22 LOG_ERR: [gnusocial.club:30144.619a816f GET /notice/1321478] Failed WebSub push to http://tgp.jp/api/subscriptions/381 for https://gnusocial.club/api/statuses/user_timeline/1.atom (HTTP_Request2_ConnectionException): Unable to connect to tls://tgp.jp:443. Error: php_network_getaddresses: getaddrinfo failed: Name or service not known; discarding
See reference: https://www.w3.org/TR/activitypub/#follow-activity-inbox
Additionally non-existent followed up users, shall be marked as such, but they shall not be deleted by system, as to leave to users record on who was followed back in time. They can be deleted by user.