Combo decks like Azorius Omniscience are something we want to exist in Standard, but when too strong, they warp the format around them and reduce the number of viable decks
Finally poor poor Putrefy, a card once so staple and powerful that has befallen many a tragedy so that it now sees almost no play
Otherwise, each creature blocking or blocked by this creature gains first strike until end of turn
And it has myriad for ETB triggers to copy like in color Mulldrifter or smashes like in color beater Deep-Sea Kraken
Legacy is not the reason, but the reason is related to why the card is good in Legacy
While the "it's slow and bad in a lot of matchups" argument might be reason enough to consider releasing [[Umezawa's Jitte] from Modern jail, it would at the very least show up in sideboards to punish people for playing small creatures