There is a major difference between 1e and 5e in this regard. As noted, in 1e level differences were expected because of xp differences between classes. A level X thief and a level X magic users weren't "equal level" in the way we think of it now.
In 5e terms, I would never want to DM mixed level groups myself, I don't think there is any benefit there. That said, I certainly could, I don't think a level difference is that big a deal....depending on where it happens. A lvl 2 vs a lvl 3...sure that's fine. But if most of the party is level 1 vs a level 2 pc that's a major difference. Or level 4 vs 5, level 5 is just a massive uptick in PC power that it would be seriously noticeable.