After Season 3, Twilight automatically stops sending letters to her mentor but then when she becomes an alicorn she officially never writes her friendship letters as told and that's why she had created the friendship journal by getting inspiration from the royal sisters journal. Celestia would always write to Twilight still but she doesn't seem to bother writing back.

I will be writing a bunch of one-shots with multiple pony characters sending letters back and forth with each other as I had a lot of fun with the first one. However, I'm having some difficulty trying to come up with ideas on more to write, so if you have any ideas, please feel free to comment them!