One of the good things about a game that is about storytelling and shares elements with childhood games of make-believe is that the fiction is very mutable. It's therefore typically very easy in my experience to come up with a plausible in-game reason a PC is absent, if you need one, or how the PC fades to the background and is still making the odd comment and helping in various scenes, but isn't mechanically impacting anything (or being impacted mechanically).
Due to using a player pool system to run my campaign, my player roster can change from week to week as can the characters. So I have a lot of experience dealing with this. Tonight, in fact, the insane dwarf clown bard who was with the party last session will not be with the party this session and his seat was taken by a paladin who thinks he's becoming a god. So the clown, Bo Low, will either fade to the background or we'll just say he couldn't stand another moment in the idyllic pastoral community of Ecstasy and had to get back to grimy Sigil. Carl Lagerbelly, the paladin, meanwhile will just turn up from Sigil having caught up with the party and he'll have a reasonable excuse for why he was delayed and missed the debate and battle in the Philosopher's Court. Bo Low will earn no experience tonight, but he also has no chance of being slain by the lurking villain in Ecstasy, nor any of the bad guys and monsters in the Gate-Town of Curst, while the player has no control over him.