Eddie
Hey, I only use a little magic, for special effects! So people disappearing, like...I give the character a happy ending. I try to keep an ending in the back of my mind just in case, and that's even if someone doesn't want to play anymore, you know? Open ended, but happy. And then logistically, I adjust the CR in whatever the next encounter is to make up for who is missing. Sometimes I make it an RP heavy session instead. Having a big campaign take a little juggling. I want to make sure that all of the players feel represented. But there are some players who don't want to RP as much, or some who really don't love combat, so I try to give everyone time to do something. I'll have someone roll really well on a perception/investigation check and let them find a ~special thing~ that isn't special until a few sessions down the road. Things like that. And just, in general, good communication with players, establishing expectations, the usual.
Yeeeeeeah. I feel like you can retrofit some of the AD&D rules into 5e, but switching completely is hard? There were some limitations that I didn't love too, who could play what, and I remember a lot of tables for calculating hitting and damage.