Did Jeremy McKinnon of a day to remember quit?

No, Jeremy McKinnon did not quit the band. He and his girlfriend did break up though sometime after Homesick and it has been said that they always had issues because he was away on tour and the band was his life.

If Jeremy McKinnon quit ADTR, it wouldn't be ADTR anymore because his vocals + lyrics AND Tom Denney's guitar writing made it what it was.

Tom Denney did quit ADTR, though he still writes with them.