Were you working on something that changed the way Discourse gets crawled, like try to use prerender on it then?
If you check your landing pages report in GA, does it give any clues about which part of the site was affected?
For the main site, if John Muller suggested that there are quality issues, I’d go through their quality docs and ask yourself if any of them apply.
From a quick look, you have redirect chains from the 2019 site that might be longer than Google can crawl.
One candidate for a sudden penalty is that URLs from the 2019 site have 6 redirects, but Google says to keep it “less than 5” or they might not follow the redirects. That might have made it appear to Google that the old pages disappeared from the Web.
Example:
curl -sSL -D - http://flynumber.com/virtual-phone-number/united-states_alexandria_1-318 -o /dev/null -H 'User-Agent: header_bot'
The redirects are probably the easiest to fix. Instead of doing them like this:
/a → /b → /c → /d → /e → /final-destination
I’d do it like this:
/a → /final-destination
/b → /final-destination
/c → /final-destination
/d → /final-destination
/e → /final-destination
(It looks like you also have doorway pages, and automated synonymizing, but I’d try to fix the simpler problem first, since that might be enough.)