If one of my players can't make it and it's not possible to remove their character from the group storywise (meaning I'll have to control their character in combat), the character will immediately equip the "Talisman of Protection from DM Stupidity". What it does is:

• When a character holding this item dies, they fall unconscious instead and disappear into a demiplane. While in this demiplane, the character is stable at 0 hit points. When the party takes a long rest, the character will reappear near a random other player character as if they had also taken a long rest.
• If the player retakes control of the character while they are still in the demiplane, the character reappears next to another player character of their choice with 1 hit point.
• If all other player characters die, the character in the demiplane will die too. If the method of their original death would have left a corpse, the corpse will reappear adjacent to a random player character's body. Otherwise, they will disappear alongside the demiplane.

Basically, it makes them immortal unless a TPK happens.