On September 5, 2019, Stack Overflow, Inc. relicensed all content from CC BY-SA 3.0 to CC BY-SA 4.0, including all previous contributions. The network's terms of service allows for them to use the material, as users provide a non-exclusive license to allow for commercial use among other purposes. However, it also makes clear that content is provided to other people viewing the site under the CC BY-SA license. Their relicensing announcement makes clear that it was previously licensed under version 3.0. Pursuant to that license, "You may Distribute or Publicly Perform the Work only under the terms of this License."
Stack Exchange has been contacted a number of times publicly and three times directly; no response has been received from any employee, let alone their legal team. A formal DMCA notice was recently sent regarding an older post; it received no response. As such, it is the intent to file suit against Stack Overflow, Inc., initially seeking a preliminary injunction to prevent the site from continuing to distribute all content under a license not agreed to. Ultimately, the goal is to block the site from relicensing anyone's content, ensuring that only content contributed after 2019-09-05 will be under CC BY-SA 4.0. As much information as legally and contractually possible will be released. The donated funds will cover the filing fee and any attorney fees that may be necessary. Any amount left over will be donated to a non-profit, likely Creative Commons.
I have been in contact with a licensed attorney, though suggestions for additional attorneys who may be interested in taking this up will be considered.