It's not the same as before - ownership has changed and the new ownership has a history of acting in ways that are not acceptable to a large segment of the population. Not to mention, for companies like GM and other automobile manufacturers - sending money to a significant stakeholder of your competition.
Advertisers know very well that they want to be associated with things their prospective customers find appealing.
Does GM want their cars to be associated with Elon Musk?
Do advertisers want to be known for spending ad dollars owned by a wealthy egomaniac known to engage in cringey Twitter arguments and retweet right-wing conspiracy theories?
Hah - just how many dumb social media gaffes has Elon made now over the last 5+ years? He has shown an outstanding lack of self-awareness when it comes to his perception on social media. He continues to double down. I highly suspect that he has only very reluctantly deletes outlandish tweets, for example.
Despite his disdain for advertising, it would serve him well to spend some time to better understand the world of advertising, public relations and public perception.
But he won't, and as a result Twitter will go down as a flaming dumpster fire as many have predicted here.