In two Twitter posts, Google’s John Mueller explained how Google treats 301 HTTP status codes that are changes to 404 HTTP status codes and soft 404 pages.
1. If you change a 301 code to 404, it’s just a 404
Sure, if you make a 301 a 404, then it'll be a 404. There's *nothing* wrong with URLs that return 404, they don't cause problems, they just don't get indexed.— ? John ? (@JohnMu) August 30, 2019
2. A soft 404 is just a 404
If URLs on your web pages use a 301 redirect to redirect to the home page, Google will treat this as a soft 404, which is essentially a 404:
A soft 404 is essentially a 404.— ? John ? (@JohnMu) September 2, 2019
How to check the HTTP status codes of your web pages
Among many other things, the website audit tool in SEOprofiler checks the HTTP status codes that your web pages send. If there are any problems, the website audit tool will notify you. The website audit tool also checks your web pages for other things that can have a negative impact on your search engines rankings: