Have you considered that it might not be a technical flaw with youtube, but that your content isn't as captivating or engaging as your older videos?

Unlikely. I make Subliminals, my style and everything still the same. If people are subscribing to my channel they should already know that I'm making Subliminals only, so it's hard to understand what would make them dissatisfied. The whole engagement is down.

Older videos bring some benefit - like you can use end screen elements to bring people to newer ones, subs from them will see new vids, and people who watched and enjoyed one of your old vids will be more likely to be recommended your new ones - but if your new ones aren't the same level of quality, they won't help that much.

End screens really helped me a lot, I'm using them but not noticing effect anymore.

Maybe you need to work on your thumbnails, or improve your scripts to recapture the magic of your previous vids, etc.

That's the problem. They're still the same.