Update:

I think I have "fixed" most of the "soft 404" errors in Google Search Console by modifying the code and content. The revalidation is slow going, but so far the "soft 404 errors have dropped from around 7000 to around 2000 and Google says 'Looking Good" as they slowly revalidate.

I also added code in the posts code block to overflow (add scroll bar) where users have posted long lines of code without code tags. There are too many posts without code tags and all these posts caused problems with Google's mobile validator; and then Google drops them from the index for "not mobile friendly" .

I also added the post summary to similar threads so Google now does not flag pages as "soft 404" for "thin content" when there are a number of similar threads listed.

Also, looking at the Google search ranking, for the keyword "unix", I noticed that our site is not the only site which has dropped. A number of older sites, including the Open Group, who used to have two pages in the top ten, have dropped even lower than our site.

Also stackexchange for the same "unix" keyword used to be high on the second page, now it hangs around the bottom of page four.

So, it seems like changes in Google's algorithm has effected many sites, and not only this one. Many have benefited and others have fallen from grace.

Let's see if the changes I made reverse our downward trend, or as I suspect, we are just an "old and long in the tooth forum site" falling out of favor with Google because our content has "aged" over the years and there are lots of new sites with more modern formats coming up on the net who also have a much strong social media presence.