The only question on MathOverflow Meta about this that I remember is this one: Vanishing questions. It contains a screenshot (in the old look).
> By default, questions with these tags are shown faded; they are still visible but less prominent. If you select Hide questions in your ignored tags in the Tags section of /users/preferences, they are hidden completely (this is done by a client-side script however, so you may notice odd “gaps”.)
Although I cannot find that setting when I look into my preferences. Maybe the interface has changed. Maybe I am just not that good in finding things.