https://www.indiewire.com/2021/01/jon-favreau-mandalorian-luke-skywalker-cameo-not-planned-1234612346/

“No,” Jon Favreau said in an interview with the Writers Guild of America. “The story unfolded as I wrote it. The Mandalorian inherits a great deal from existing Star Wars stories, and when I write, that context is always a consideration. It became clear that, within the established continuity, certain things were likely to transpire.”

I was so shocked to see Luke at the last episode! I didn't expected it!

The Mandalorian reveals how it kept Mark Hamill's Luke Skywalker cameo a secret (https://tvline.com/2021/08/25/the-mandalorian-mark-hamill-season-2-finale-luke-skywalker-returns/)

A special episode of Disney Gallery: The Mandalorian showed how the Season 2 finale used Hamill and double/stuntman Matt Rugetti to bring Luke Skywalker back to life. The special also revealed that the script for the episode used the name "Plo Koon" in place of Luke.