Previous campaign: the PC is presumed to still be there, but “off camera,” and fights were rebalanced so their fair share of monsters are also off-camera.
Current campaign is a very silly megadungeon called The Grande Temple of Jing. When players are absent, a slot in the floor opens and their characters are literally get sucked into the floor like it’s a pneumatic tube. When the player comes back, they’re ejected from the ceiling in the same way, covered in slime, and must make an Acrobatics check or take falling damage. (Sometimes, characters return in the middle of a fight, and with a high enough Acrobatics check, land on top of the monsters for a free attack.)
While gone, they’re in stasis and don’t get the benefits of rests. When the players turn in money for experience, which is the only way to get XP, it’s divided among the PCs that are present. There’s no attempt at balancing fights, let alone rebalancing them… players have learned to be cautious.
One player dropped out, so next session his character was sucked out of his clothes, Looney Tunes style, leaving all his possessions to scatter on the ground for the rest of the party to collect.