So, I was watching a video of an acoustic performance from a certain artist on YouTube. Decided that I really liked that version. I searched for that version in the tab DB but could not find that particular versoin. I attempted to post the link in a request thread in the Tab Talk forum. It showed up as a forbidden link.

How come we're not allowed to post links to youtube? I don't see anything in the forum rules about it. Sorry if this question has been asked a lot here, but I just want to know why.

It's a measure introduced to reduce certain kinds of spam but they didn't tell anyone. You need to get to a certain number of posts before they'll let you post links, I'm not sure if it's specifically youtube or links in general. I also don't know how many posts it is.
R.I.P. My Signature. Lost to us in the great Signature Massacre of 2014.