TYPO3 Core

Respective a) and b):
I have to disagree. Image the following scenario: You made a relaunch with a new site structure, all pages, that are not need anymore, should show a 404 (like old news and such stuff). Instead they are redirected with 307 to the home page. If a user comes from a search engine, he is confused because he doesn't see the correct content. Search engines are confused, because they "think" the content of the original page should be left indexed. But, search engines should throw the indexed page out of the index (the content isn't available anymore). So you might insist, okay, we are making a 301 permanent redirect. So the original page is unlisted by search engines. But this is discouraged by Google [1].

Also, if a user clicks on the (original) link in a search engine, then he sees the home page, not the desired content, and leaves. I have a short click (hurray) and if this happens more the once, this is a signal to Google to rank this page down (it is not relevant, oh wonder).

Another scenario: Someone links to our page. Then the editor of the other website doesn't get noticed, that the original page doesn't exist anymore, because a 307 is okay for the link checker.

As a side effect: I don't see the "wrong" URLs in Google Search Console (it is not a 404), so I cannot make a manual permanent redirect to the correct page (if I missed it before to do that).

So TYPO3 makes an assumption (all URL not defined by a language prefix have to be temporary redirected to the home page of the default language) which is no good.

Respective c):
This bug was fixed in a 9.5 update, in 9.5.4 everything is fine. I updated the description.

[1] https://www.seroundtable.com/google-redirect-urls-home-26270.html

Hey. Thanks for your feedback.

One solution in your case might be to skip the /en for your base path and use / for default language. If then /foo is called, you'll receive the 404 response with your english 404 content.

Another solution if your link layout changes, colud be to 301 old urls to new content urls (not just the homepage), for instance using the redirect module. This gives you better user experience and the ranking of your old page location is transferred to the new location.

If a domain+language like www.example.com/en is configured as default language, so base path www.example.com/foo is not configured (there are typo3 instances act on subpaths like www.example.com/bar for the domain part in default language, having www.example.com/bar/de for a translated page in this sub path), then i guess it's hard to guess what should be rendered: You expect to get the default language 404 page rendered in this case, however I'm not sure if that can be easily achieved in the router since that would require some guessing and chances are this may introduce unexpected side effects for possible sites in a sub-slug. I suppose a patch in this area would have to be carefully crafted and tested. I also guess it varies what people expect to happen in this case.