Icahn potentially isn’t abandoning Trump based on what Trump stands for as much as he’s just saving his own tail. In general, he still supports what Trump is doing by slashing government regulations, but wants to avoid his own personal political & legal quagmires.
Sure, Icahn gave Trump advice (he endorsed hiring Mnuchin, Wilbur Ross, & Pruitt) but I’m not familiar with anything Icahn exclusively did to protect Trump as President. In other words, what “protective power” did Icahn provide for Trump that is now lost?
Here’s a long and interesting deep dive on Icahn (also linked in both the Globe & Mail and Forbes articles).
A few excerpts:
Icahn’s role was novel. He would be an adviser with a formal title, but he would not receive a salary, and he would not be required to divest himself of any of his holdings, or to make any disclosures about potential conflicts of interest. “Carl Icahn will be advising the President in his individual capacity,” Trump’s transition team asserted.
(The day Icahn’s appointment was announced, he made half a billion dollars on rising shares of his stake in his company.)
Here was a more appropriate job title for Icahn: 
It was hardly lost on Wall Street that the famously single-minded investor might leverage his new role to advocate for his own investments. Barron’s asked, “Has Carl Icahn been appointed Secretary of Talking His Own Book?” The Web site Dealbreaker, noting Icahn’s lack of conflict-of-interest constraints, proposed an alternative job title: “Secretary of Do Whatever the Fuck You Want.”
Part of a backstory on his relationship with Trump since the 1990s…
Icahn and Trump maintained a loose friendship during the ensuing decades, one that was hardly as intimate as Trump likes to make it sound. The very notion of a relationship that transcends mercenary self-interest may be alien to Icahn. Once, in a court proceeding, he said, “If the price is right, we are going to sell. I think that’s true of everything you have, except maybe your kids and possibly your wife.”
“Possibly?” the judge asked.
“Possibly,” Icahn said, adding, “Don’t tell my wife.”
(Icahn ended up getting a divorce 
)
Icahn is just as self-serving as Trump. I think now that Trump’s presidency isn’t accomplishing as much as Icahn thought it would, the risk-to-reward ratio is too high—not just risks from Trump, per se, but risks of his financial empire taking a hit under intense public scrutiny in general.
Trump’s bid for the Presidency may have seemed like a long shot, and Icahn professed to have misgivings about the “negativity” of his chosen candidate. But he believed that Trump would slash regulation and, specifically, would make the change that Icahn wanted on biofuel credits [to benefit his refinery company].