I try to keep all of my villager's goodbye letters, regardless of whether I liked them or not. I want to remember who hasn't been in my town and who has, that way I can get villagers that I haven't had in Bluebell yet.
I started keeping my letters. It's nice to look back on memories, even if I didn't like that villager too much. I wish I had done this way earlier, when a few of my dreamies moved out tragically. I usually save other nice letters that I get from villagers I like, or letters from special characters.
I save a lot of letters ranging from Goodbye letters to silly letters from my dreamies and letters from my mom. I feel like the Goodbye Letters keeps the memories of my town intact; I managed to get their picture from a few. In general, I have the most letters from Stitches; his never get old.
I've kept goodbye letters from villagers that I either intended to keep or that I decided to allow to move out despite liking them. I generally toss out the ones from villagers I didn't really like.
I like to keep them both for the sentimentality and because it's a nice way to store items you can't fit anywhere else. Just throw the items in the empty letters and save them in the post office until you want/need to use them again. Yes, I'm a terrible hoarder in this game.
I keep goodbye letters, even if it's from a villager that I didn't like. It's always nice to look back and remember how fun it was to have them in my town.
I have started to in Lunavila, but not at first, so I don't have every villagers goodbye letter, and I've almost got every villager that I want to be permanent.
In my Wild World town, I have kept every single letter.
Luckily I didn't have to kick many villagers out, but I keep Pecan's, Ricky's and Tank's goodbye letters because I liked them. I didn't keep Becky's, Filbert's or Clay's because they were unwanted.
I used to keep all of their letters in the previous game, Animal Crossing W/W. But I realised that I didn't really look back on them enough so I decided to stop. I only really keep goodbye letters of the animals that I really liked. Most of them being either Kranky or Peppy!