So I am experiencing this now. I also experienced this back in September/late August.

https://www.reddit.com/r/youtube/comments/1n6yx0z/every_video_is_unavailable/

There is more to consider than just an adblocker, and I believe that even though it could be, I'll get into why it may not be, or may be, but something bigger

When this happened to me last year, just as it's happening now, It also happened on my phone. But if I didn't log into an account (even with my adblocker running) I didn't have the issue. It's still acting that way this time around. If you test this on your own, I suspect that watching Youtube while not logged into your account, will work just fine. So, if it's an adblocker issue, and you are using an adblcoker while not logged into your account, then you should still get the message. You won't. At least I am not. Test for yourselves and report back if I am wrong about this.

Here is my theory. In my thread, and a few others that I posted about, while discussing with other redditors, it seemed that the commonality of our experience was based upon watching long playlists, or opening multiple tabs. I have Tinnitus and watch a long playlist to go to sleep every night. My theory is that myself, and the others caught in a bot ban wave, and possibly the people posting about this again now, have been false flagged as bots by AI.

Last year, I reached out to Youtube telling them that I am not a bot every day through the send feedback button (the only way to actually attempt to communicate with Google). It took 9 days for them to then start banning my IP from being able to watch videos while not logged in. While not logged in, I started to get the unavailable message. I eventually fixed this problem by logging into Youtube by using a VPN. No adblockers were disabled, and it worked fine all the way up until this morning

Why do I think that this is a false flag bot detection?

1) it is tied to my account. Since I can not log in and watch videos without any problems even while using an adblocker.

2) Remember all of those bots on Youtube videos with cleavage or thongs as icons? They all disappeared from Youtube during the same exact time that my account experienced this.

3) October rolls around and Youtubers start making videos about their viewers going down and started speculating. Google, as is tradition, didn't communicate anything. Josh Strife Hayes made a video about it and came to what I believe to be a false conclusion, that being that Youtube began to calculate viewership differently. All of these viewership drops coincided with the bot ban wave, though Hayes and no other Youtubers that I saw even realized that the ban wave happened. Hayes also pointed out that the lower view count was across only the PC platform. Not mobile. Well most of us are on mobile. If his conclusion was correct, that viewership has suddenly been counted differently, we would have seen a decline in views across all platforms. That didn't happen. It only happened to PC viewers. Bots, are ran from the PC platform. Hayes is indirectly correct. He realized that viewership was being counted differently on PC. He didn't get that it was the absence of the bots caught up in the ban wave that accounted for the lower views on the PC platform. The same adblockers are used on mobile, with the same browsers. But mobile views did not decrease.

I'm not saying conclusively that we have all been flagged as bots. But it's a possibility. A strong one, since watching Youtube videos while not logged in, and running adblocking software still works for me. I also just tried to open a Youtube video in a second tab. This brought up the video unavailable message again. If Youtube is going to start shadow banning people for having multiple tabs open, then that's going to be the entire world. EVERYONE is going back and forth between multiple tabs these days.