If you were born the year Aven Mindcensor was printed, you'll be graduating high school soon if you haven't already.) People kind of lost their minds, because there had never been a card that messed with deck manipulation this effectively
Additionally, Slimefoot causes each opponent to lose one life and for you to gain one life whenever a creature you control dies, synergizing greatly with the sacrifice-heavy strategies that are sped up by the disposable tokens Slimefoot can produce
This Sphinx joins that number with flying, hexproof, and the inability to be countered when you cast it
I still like the card a lot due to how versatile it is but the fact that it can have a pretty disappointing floor, especially if your opponent is killing your creatures, makes it hard to play the full four copies
Red mythics are often explosive and fast, offering either some of Magic's leaner threats or its most powerful spells