The Subscriptions feed error message is resolved for almost everyone. If you're still seeing it, know that it will be fixed soon – thanks for your continued patience! You can follow along here for any additional updates: https://yt.be/help/CBxM https://twitter.com/TeamYouTube/status/1119013035580878848 …
We’re rolling out Channel memberships gradually and hope to expand it! If you’re able to add the Join button to your channel, you’ll be notified and can visit http://youtube.com/channel_memberships … to get started. Learn more here: https://yt.be/help/b3jg . Hope this helps.