I have 20+ years of SEO experience, going back before it was commonly called SEO, before Google even existed. I cut my SEO teeth figuring out how to manipulate AltaVista rankings.
Here, I've marked up a typical content page of yours, showing what's wrong very blatantly. It took about 30 seconds to diagnose the whole site based on the repeating content farm structures and thin content problems.
https://i.imgur.com/k5brlYr.png
That page maybe has 20 or 30 words of quality content on it. That will get you an extreme demotion from Google if you're running a whole site like that in a content farm structure.
Your site was very likely penalized due to low quality, shallow content. Google tagged it as being a content farm. No conspiracy is necessary, it's obvious why it's penalized. Any SEO expert would point out the countless problems in the first few minutes of a discussion with them.
I looked at numerous pages, all were of very low quality, and thin / shallow on content.
On one page I checked there were 471 words, most of which were repeating low value words or from low value segments of the page (eg the "Link to this slang definition" section). Google knows the page is thin on quality content, aka shallow. I found this to hold true across all the pages I checked.
On most of the pages I checked the most frequent non-common words were "definitions" and "include" - ie hyper low value content. Your typical page looks like it might have 50 or fewer words of quality non-repeating content (content unique to just that page, written by a human). That's hundreds of words shy of what it needs to be at an absolute minimum.
The pages all have repeating content farm structures (which were increasingly common 8-12 years ago) with shallow content in those areas of the page, which is something Google sought to clean up around the time you're claiming the penalty occurred, which makes perfect sense.
You built a content farm, with a lot of shallow content pages, Google dramatically changed how it treats those types of sites, and your site most likely got nuked because of it. If you build a content farm structured site, where all the pages have repeating segments and your offset of positive value only consists of dozens of words of quality text on each page, you stand zero chance of getting traffic. That started being true 8-10 years ago as Google updated its algorithms to target content farms, to wipe them out of existence. They killed a billion dollar site, eHow, circa 2010-2011 in the process:
https://en.wikipedia.org/wiki/EHow
