Remember when Phage use to have a chance to slow for a 2 seconds and it was removed for being OP?

It wasn't removed for being OP. It was removed because Riot was steadily removing a lot of RNG from the game.

Red buff doesn't have a chance to slow it has a guaranteed slow every hit. The slow was removed because of the random factor for phage and having the slow guaranteed would of been to good for phages cost leaving them with only one option, removing it and replacing it with a guaranteed speed buff.