The site license says that when you post code, the material is available to others under the CC-SA conditions. That means that for example you can't simply 'pull' all of the content: important so that the material remains available for others, and particularly reflects the fact that as others can edit, it's hard to say content is '100% yours'.
However, you retain copyright on material you post that you wrote. In particular, with code, provided it's 100% material you wrote, you remain free to use it under an alternate license. That's for example how companies offer GPL and commercial license versions of code: the copyright holder is not restricted by the license, it's other people who are.
Conclusion: you are free to use your code how you like. It's everyone else who has to respect the CC-SA (unless you've given them explicit permission otherwise: for example, I view my contributions as 'public domain' so make my code available as CC0). So you can use in closed source, re-license as MIT, LPPL, GPL, ... or whatever.
As I say, you are copyright owner. You could therefore re-post your material elsewhere on a site you control. For example, I have various postings to my own blog, where I set the rules and which is explicitly CC0 (see https://www.texdev.net/about/). If you do that now, you'd be be able to show from timestamps that the material was posted before any account removal here.
To be clear on the copyright part, the terms say
All materials displayed or performed on the public Network, including but not limited to text, graphics, logos, tools, photographs, images, illustrations, software or source code, audio and video, and animations (collectively “Network Content”) (other than Network Content posted by individual “Subscriber Content”) are the property of Stack Overflow and/or third parties and are protected by United States and international copyright laws (“Stack Overflow Content”).
They then go on to describe Subscriber Content:
You agree that any and all content, including without limitation any and all text, graphics, logos, tools, photographs, images, illustrations, software or source code, audio and video, animations, and product feedback (collectively, “Content”) that you provide to the public Network (collectively, “Subscriber Content”), is perpetually and irrevocably licensed to Stack Overflow on a worldwide, royalty-free, non-exclusive basis pursuant to Creative Commons licensing terms (CC-BY-SA), and you grant Stack Overflow the perpetual and irrevocable right and license to access, use, process, copy, distribute, export, display and to commercially exploit such Subscriber Content, even if such Subscriber Content has been contributed and subsequently removed by you as reasonably necessary to, for example (without limitation):
importantly saying non-exclusive, which can only reasonably be interpreted as the author retaining copyright.