As mentioned, I have done a lot of checking, and we did not make any changes in March of 2018, as I was busy working on another project (cybersecurity).
I have no idea what caused Google to drop two-thirds of our links from their index.
One of the reason(could be not filly sure) that sometime back we renamed a FORUM from "unix-for-dummies-questions-and-answers" to "unix-for-beginners-questions-and-answers", I apologies if I am wrong at that time I remember many links were broken which you fixed(this could be internal fixes for forums too not sure about it) but thought to mention it here. Apologies if I am wrong here.
EDIT: How about posting this in "GOOGLE forums" too? Till then I am also checking which place should be proper for this kind of question.
No. That's not the problem I don't think. We ended up not renaming it, but creating a new forum so the old forum would not break links.
However, I agree this could have contributed to the problem.
As they say "the enemy of working good is 'make it perfect' .. ." LOL
We definitely broke something OR Google changed something in their algo, but even if we did, Google would / should reindex it.
We need SEO expert and it's best not to speculate, as we really need an expert who can look at the site, sitemaps, index, meta tags, etc and know for sure what could have happened.
In other words, we need an expert, not guessing and speculation, LOL
Note: I posted this question to our SEO software provider, DBSEO today as well: