Losing followers
Followers might die, of course. But they might also stop being followers for a number of reasons.
The relationship between follower and PC might be temporary or situational.
When it no longer makes sense for them to take orders from the PC, they stop being a follower. They might return to follower status again, but for now, they have their own life to live!
Fflur became Blodwen's follower on their trip to Gordin's Delve, but after they return home, that doesn't really make sense. The publican's son takes care of the horses, Fflur and Celyn, and they're often out in the fields.
Should the PCs bring Fflur on another expedition, we'll likely treat her as Blodwen's follower again. But around town, she's just a horse that Blodwen is friendly with.
A follower might change in a way that they no longer follow their PC. Maybe they lose their nerve, or move away, or have kids and settle down, or so forth. They might still be around as an NPC, but they're no longer a follower.
Eira survives her brush with death, and Blodwen heals her wounded soul, but she's never the same. It's like she aged 20 years in a week, and the next session, she tells Rhianna that she's done. It's an emotional scene, and we actually see Rhianna cry. When it's over, she erases Eira from her Crew insert and I add "retired" to her entry on the steading playbook.
Of course, it's not always amicable when a follower parts ways with their PC. If the breakup is messy, then the follower might very well become a threat. If the follower has been harboring growing resentment towards their PC, they might even betray them in sudden and dramatic fashion.
Vahid's put-upon wind spirit returns from spying on Thornthumb, full of news and playful energy—right as Vahid sits down for dinner with the widow Maire. He steps outside and pleads with the spirit to behave for now, and promises to play the flute out in the fields later, so that the spirit can dance and frolic.
The andalau is out of Loyalty, so Vahid rolls to Persuade. Alas, he rolls a 4. I ask Vahid how he thinks he screws this up, and he says that he loses his temper and starts shouting, trying to bully it into behaving.
I decide that the spirit has had enough. "It gets quiet and still. Then, hoo boy. The wind picks up, more and more, getting stronger. Gods, you didn't think it could be this strong. It howls, screams, and snatches at the flute." Vahid tries to hold on, but rolls a 3 to Defy Danger, and the flute gets smashed apart on the Stone.
The spirit was tethered to that flute, and now it's not. It's certainly no longer a follower. An untethered spirit would normally dissipate, but I think this one holds a grudge. It vanishes for now, but between sessions, I write it up as a threat (a magical entity*, Instinct to torment Vahid).*