A 404 error is a perfectly normal and valid response code from a website page that says "this page no longer exists".
A soft 404 is applied when Google reviews a page and essentially draws its own conclusion that the page has gone away, even though its not getting an actual 404 response on that page.
For IDX, the soft 404 is coming from pages that were indexed, are still on your website as valid pages (they are not responding with a 404), but now show a message that says "This listing is no longer available".
Google will probably see this message on a lot of pages - and is concluding that the content has gone away, and so applies a soft 404.
Where these are generated by property pages, it will be because the property has been removed (not sold - but actually de-listed) from the MLS and so is also no longer in the feed.
We have been aware of this issue for a while and we continue to monitor feedback from all our customers on this.
We currently have no workaround, although the recommendations we are aware of include -
- Do not attempt to apply 301 redirects to such pages, Google does not like this
- Applying a hard 404 response to indexed listing pages like this is the best approach
So, in summary - soft 404 are not a problem - just product pages that have gone away.