OK.
I have rechecked three dozen or more links that Google Search Console had listed as "soft 404" errors, using three different HTTP header inspection tools (using Chome browser):
- DevTools (Chrome 73)
- HTTP Header Spy 2.0.49
- HTTP Headers 1.1
None of them show any 404 errors of any kind, hard or soft; and all links are providing the correct 200 responses. Some of the URL rewritten by DBSEO go
302 -> 200, but this is technically correct.
As far as all the tools in my toolbox, we are running as we should.