History of mega-collaborations
According to this article from 2012, papers with multiple hundreds of authors began primarily as a phenomenon in physics, and the massive physics collaborations in areas like particle physics and space sciences continue to be the ones expanding the bounds of mass authorship.

Historically, the post notes:


  In 1981, the highest number of authors on any paper indexed by Clarivate Analytics was 118; by 1989, the annual number regularly surpassed 500.


Another article notes, however, that despite the overall dominance of physics, the first article to pass 1000 authors was actually a 2000+ author medical study.

None of the sources that I have found talk about any particular difficulties in publishing multi-hundred author articles, which does not surprise me. Many of the issues, such as assigning responsibility, already had to be worked out for articles with "mere" dozens of authors. Moreover large-scale collaborations are typically difficult to conduct (and fund) without already having effectively satisfied many of the challenges of peer review.  In short: while I don't have evidence for it, I would suspect that the early mega-collaboration papers were well received by journals eager to publish such high-profile and likely impactful research.