In season 9 episode 9 The Platonic Permutation Howard meets Elon Musk and Elon says he wants to stay in touch.
Did Howard ever hear from Elon Musk again? Or didn't that potential storyline get any further showtime this far?
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Till season 11 episode 15, Howard didn't hear from Elon Musk back. There is no in or out-of-universe explanation for this, so maybe that storyline didn't get any further showtime this far.
However, Elon Musk gets mentioned again by Sheldon Cooper in The Military Miniaturization (S10E02) while discussing about Elon's theory that our reality could all be part of a video game played by aliens.
Sheldon: Look at this. Elon Musk has a theory that we’re all just characters in some advanced civilization’s video game. Source
He appears in Young Sheldon S01E06 (A Patch, a Modem, and a Zantac) episode as well, where he
manages to create a landing rocket successfully through reading from Sheldon's notebook.