Good people remember though and hold a grudge if you insult them on the way out the door. People you want to hire in the future are watching as well. There's incentive to be classy (ex: Microsoft's https://blogs.microsoft.com/blog/2023/01/18/subject-focusing...) unless the whole ship is going down.

Edit:

That being said, after actually reading it, the Meta letter is relatively classy. The HN comments made it seem worse than it was.