The Purple Cow of Death in RPGs

RPGs are played by human beings, and human beings have lives outside of the games they play. Players cannot always be there for every session.
There was an old joke we picked up from another group of D&D players that had a very strict rule that if the player wasn't there, his character wasn't there.  The Purple Cow of Death would fly in and swallow up characters who's player is missing and fly back to spit characters out if the player comes back or a new player/character joins.
No one questions the Purple Cow or her actions, and no one questions the missing or added characters.  You just act like the character was always/never there. That is how the literal Purple Cow works.
I don't like to go that extreme, and have characters disappear and reappear in the middle of a dungeon especially if the character's player is only missing temporarily and is not leaving the group for a long period of time.  I will see if someone is willing to play two characters or maybe the Game Master can take up the missing player's character.
That said, sometimes it is necessary to write someone in or out, so I will try to come up with an explanation.  It may be a very a flimsy excuse, but there will be an excuse, and people will not question the flimsy excuse for why a new character joined. There is an in-universe story explanation, it's just not a very good one. This is the metaphorical Purple Cow.
I try to have new characters enter or leave at the start of new story arcs.
For instance, I ran my campaign "We Have No Class" with just Aranil the elf and Neshik the gnome.  At the beginning of a new story arc began in the city of King's Lake, a new player joined in with Svetlana the half-orc, so we said that they were already acquaintances and just happened to have re-met in King's Lake without probing deeply into how unlikely this was.
The Purple Cow can also be used if an aspect of the setting is retconned, the players and their characters are supposed to act like this new thing has always been there.