Some VFX Shots Were Finalized
Shortly after Justice League arrived in theaters, a number of deleted scenes were leaked, many of which showed incomplete visual effects. This led many to believe that, since Joss Whedon had been working on the movie since before the reshoots started, Snyder didn't have enough time to complete visual effects, but another social media post from the director, this time on Twitter, actually proves that a significant amount of VFX work had been completed.
The post included a video of Jason Momoa's Aquaman underwater, but the biggest reveal in the video isn't actually a part of the movie. On the brief splash screen at the start of the shot, some text reveals a number of details about the scene. First is the date: February 27th, 2017. This means it is from long enough after the DI work that the color grading for the entire movie could have already been completed. The second detail is the text at the bottom, saying "DPX for final per request. Original submission not [obstructed] -rious internal [obstructed] review as proposed final."
DPX is a Digital Picture Exchange file, which is used for visual effects and digital intermediate work (again confirming picture lock). As it says at the end of the text, this specific file is for "review as proposed final," meaning Snyder was looking at this shot to possibly approve for the final cut. This indicates that this scene, and likely a number of others, had completed (or almost completed) VFX.
The leaked deleted scenes were also pointed to as evidence of incomplete VFX, but even some of those shots had already shown up in trailers with more polished CGI, meaning the leaks weren't of the most complete versions of the scenes in question. It's not uncommon for bite-sized portions of some VFX shots to be finished for inclusion in a trailer, so this doesn't guarantee the entirety of those scenes were completed, but it does support the assertion that a significant amount of VFX work was well underway.
Audio Mixing Was In Process
We know Justice League certainly reached picture lock under Snyder and VFX work was far further along than assumed, but what about audio? Well, again, thanks to Snyder's Vero, we know Gal Gadot completed ADR (automated dialogue replacement) for her scenes on January 27th. If Gadot is doing ADR, then the rest of the "Foley" (ambient, character, and environmental sound effects) were also being completed. ADR is another task that happens after picture lock, so this actually bumps that timeline up by a month, meaning an extra month of post picture lock work for Snyder than previously mentioned.
The haziest area of production is likely the music and Tom Holkenborg a.k.a Junkie XL's score. While the composer had clearly put thought into what he was going to do, and would presumably maintain some continuity from his work with Zimmer on Snyder's previous two installments, it's not clear whether or not he actually wrote or recorded any of the soundtrack before he was fired. He did collaborate with Gary Clark Jr. for the 'Come Together' cover featured in the trailer, but per a video on his YouTube channel prior to his removal from Justice League, he suggested he didn't plan to start recording for the movie until June or July 2017.
A number of songs have circulated, pulled from the trailers or otherwise, that fans claimed were Junkie XL's original score, but the musician sent out a tweet to clarify that he didn't create any of the "leaked" music. While many elements of post-production have evidence pointing to the amount of progress seen, the soundtrack is the biggest question mark. During the editing process, directors typically insert temp music to simulate the desired final product, meaning the Snyder Cut could have been compiled with a collection of Junkie XL's past music, particularly from the other DC projects, so, if he truly never recorded the soundtrack, it could be watchable with a sort of "finished" music, but the music wouldn't be new or catered to this edit as Holkenborg's soundtrack would have been.
Page 4 of 4: Why Was The Snyder Cut So Far Along?