GM's official statement is "We are engaging with Twitter to understand the direction of the platform under their new ownership. As is the normal course of business with a significant change in a media platform, we have temporarily paused our paid advertising. Our customer care interactions on Twitter will continue".

Is it a publicity stunt? or is it Virtue signaling? or just like they said, "the normal course of business"?

If you look at Twitter, nothing actually changed, it is almost the same, except the fact that the company is now owned by a GMs "competitor". I'm using the word "competitor" loosely here because the US EV market is dominated by Tesla, nobody is even close. Take a look at this infographic on the US Electric Vehicle market so far in 2022:

Has any other company pulled out after Elon Musk's acquisition? 

Whatever you say about Musk and his antics, at the end of the day he is a businessman. He did not buy an overpriced company just to restore democracy or free speech, he bought it because he likes the platform, and he has a plan to make it profitable.

People need to stop looking at Musk as an impulsive guy, or as the savior of humanity, both views are idiotic. To me, he is a remarkably intelligent businessman who has some audacious goals in life.

Coming back to the topic, I really think that GM paused its ads on Twitter because they are worried about the platform being overrun by "right-wing racist trolls", and they don't want to get canceled for "funding white supremacists". I know, I sound like most of the far-left media right now, but that's what I think led GM to suspend its ads on Twitter (and for some virtue-signaling brownie points).