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Where in the document does it say that. not in the introduce yourself section.

I had to go and check myself... I thought it did, but now I remember that it was other articles I read that suggested I only use it once. I used the tag twice. Then some tag abuse bot mentioned me and I stopped.

It seems that people decided a long time ago to give the introduceyourself tag to just fresh accounts so they can get exposure. Of course, there are much worse offenders and there are silly rich developers that will create a new account about some new app they made and use the introduceyourself tag and upvote it to trending.

I never see the really big accounts get messed with by the bots at all. With so many new user's it is a good idea not to use it though.

While I don't fully disagree with your incentive here especially since you only informed and didn't flag, what you're saying here is silly. Have you even read the Welcome document yourself? It doesn't say that you can only use it once, it only says that it's tradition for new users to create an #introduceyourself post.