That's against the Youtube TOS and can get your account suspended.
On to the hidden tag development...
Metadata is important in getting your video seen in search results. Good title, description, and yes, tags. Without that stuff, good luck in getting views... hidden tags or not. There's still a field to enter that information. It still works.
Anyway... I'm glad to see they've hidden them.
For starters, it'll help in killing off the reply girl plague and similar blatant tag robbery.
On the other hand, it will make it harder to find your own videos someone may have ripped off your channel, in it's entirety, for use on theirs. Tags are how I found one of my videos someone had reuploaded and monetized. Smart-guy just copied and pasted all my tags... my channel name included.
Brilliant.
They stole everything else too... links to my other crap and all my links to my other videos. Pasted right to their (my) video. Thanks for the free... uhh... advertising?
Absolutely brilliant.