According to The Hollywood Reporter, he was found dead in his apartment.
When you think of the people that created Spider-Man or Doctor Strange, the mainstream think of Stan Lee, but few mention their co-creator, Steve Ditko. While Stan is the face of the brand, Ditko worked behind the scenes, and it started in 1961 when Ditko re-designed what we now know as the look of Spider-Man today.
Not only is he credited with this, but some of the other characters as well. THR shares that he also created some of the villains we're aware of in the Spidey-verse, including Doctor Octopus, Sandman, the Lizard, and Green Goblin. And then 1963, he was tasked with Doctor Strange.
The unfortunate end to Ditko, according to THR is that "The New York Police Department confirmed his death to The Hollywood Reporter. No cause of death was announced. Ditko was found dead in his apartment on June 29 and it is believed he died about two days earlier."
As far as any response we may hear from Stan Lee on the subject, it might fall silent. According to the report, the pair have not spoken in quite some time, and the reason isn't of public knowledge. THR also says "The best explanation suggests Ditko was frustrated at Lee's oversight and his failure to properly share credit for Ditko's contributions to Spider-Man and Doctor Strange."
After leaving Marvel, he did some work for DC Comics as a freelancer, but his most recent contribution for the comic world for Marvel was Squirrel Girl in 1992.
He also hasn't ever been known to be married, and no known survivors. They say that he continued to write and draw, creating until he passed, so there might be many unreleased levels of material in his Manhattan studio.