If you haven't noticed yet, YouTube has removed the "All Activity" link from the Subscriptions pages. That link was a great way of discovering new video postings from sources other than your selected subscriptions, and now you're left solely with the standard Search box if you're hunting for something new. If you complain (as many of us already have), you'll see YouTube's rejoinder:
"Thanks for the feedback; we'll pass that onto the team."

If you want to add your complaint to the pile, you can submit via the YouTube page at Help>Submit feedback.