Elon Musk’s “thanks for being awesome”
A closing remark is called a valediction.
"Thanks for being awesome" is certainly informal, but not irreverent. He probably closed the letter in this way to establish that his decision was reached with a certain element of goodwill, especially considering the previous sentence (Telsa is staying public because it's probably in the best interests of its investors --- and it was awesome of them to choose to invest in Telsa, so thanks).