The Sinister Six film was in the works way back since The Amazing Spider-Man was a thing, but after Sony decided to reboot the character, it looked like S6 was dead in the water. As it turns out, the movie is still in development, and it's just a matter of time before it actually gets made.
Talking to Vanity Fair, Sony's Amy Pascal confirmed, "I'm just waiting for Drew [Goddard] to be ready to direct it… I would do anything with Drew Goddard. I'm just waiting for him to tell me he wants to."
The original plan for Sony was to have Sinister Six release between The Amazing Spider-Man 3 and 4, but seeing as that franchise is no more, S6 had to take a step down for development and wait for Sony to smooth things out. Not to mention, Goddard is now attached to directing Fox's X-Force movie—if that's still happening that is.
With Marvel Studios doing a great job balancing the villains in their Spider-Man films, I think they're actually building up to a Sinister Six team-up in the third Spidey MCU film. For now, we already have the likes of Vulture, Scorpion, The Tinkerer, and Shocker in this universe; not to mention Donald Glover's Prowler as well as Far from Home's Mysterio. I think another team-up in the SUMC would just ruin that momentum.
Though Venom did make a lot of money, the film is far from a critical hit. It's only a matter of time before audiences won't be eager to dish out cash for movies of the same caliber. Best for Sony to stick to movies like Spider-Man: Into the Spider-Verse which turned out fantastic.
No release date has been set for Sinister Six, but catch Spider-Man: Into the Spider-Verse in theaters now.