Google's John Mueller said that spammy links to your site would not be the cause of soft 404 errors being displayed in Google Search Console. Honestly, I've never heard of this theory before, so it is new, and I figured I would share it. But I do not fully understand why someone would think spammy links would cause soft 404s.
I guess that if your 404 handling is not done correctly and someone posts a ton of fake links to your domain name but not to real URLs on your site. Then Googlebot tries to access those links and then sees a soft 404, instead of a real 404. But you should be serving up real 404 errors for URLs that do not exist on your site, you should not be serving a 200 status codes, they should be 404.
John said on Twitter "No, spammy links don't cause soft-404s."
Here are the tweets:
Google's Soft 404 bug is back @JohnMu @searchliaison? I'm seeing huge spike across multiple domains. Unique pages disappear, URL inspection tool is hit or miss "Something went wrong. If the issue persists, try again in a few hours". Was this resolved completely for you @jsilton? pic.twitter.com/qmJMTJJ3XF
— Tom Rothwell (@trothwell_null) June 30, 2021
No, spammy links don't cause soft-404s. I'd consider posting the details in the help forums, so that folks there can take a look and potentially give feedback.
— 🍌 John 🍌 (@JohnMu) July 1, 2021
Forum discussion at Twitter.