They scrapped them for the very sensible reason that the team that is away in the Second Leg (IE the team that finished lower in the league) could well have an extra 30 minutes in which to get an away goal. Folk eventually cottoned on that this was deeply unfair.
Ok. I think a fairer system would be the play off semi is all square after 2 legs after 90 mins, then the team that finished higher goes through. Forget pens.