What to do with a 'defunct' sub-domain?

[Gemini23]

A sub-domain that was previously 'populated' with content from another domain - via CNAME has now become 'defunct' and the content isn't there.
There are several hundred urls indexed in Google etc... where of course if users land on one of these urls they will get a page not found.

What is the best way to deal with this sub-domain removal - there isn't a 'like-for-like' replacement to redirect to.

I can redirect the root of the sub-domain to the main domain but not sure how best to approach 'the rest'?

[not2easy]

redirect the root of the sub-domain to the main domain

Sorry, but that is a bad idea, a 410 response is ideal but even a 404 is better than redirecting to the root domain. That creates a 'soft 404' which is something repulsive to Google.

If you want to put in a simple index page there that offers an explanation for the missing content and links to other content on the site, that might help you keep the traffic. But the right thing to do is to use the 'Remove Content' tools in GSC and return a 410 server response so it will be removed from the index faster.

[Gemini23]

many thanks - the latter option seems the best course of action.

[not2easy]

I would look into that CNAME business as well, because if you want it to be treated as a defunct directory, that isn't helping.

[Gemini23]

In fact, checking - there are 25k urls indexed..
that result in,...
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator at webmaster... to inform them of the time this error occurred, and the actions you performed just before this error.

More information about this error may be available in the server error log.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

[lucy24]

Internal Server Error

That, unfortunately, is a problem only you can research. Look closely at the part of the config file that's specific to the subdomain, or the htaccess (if shared hosting on Apache). Note that you can only redirect from a hostname (subdomain or whole domain)--or return other responses such as 410--if you retain access to the host.

Additionally, a 500 Internal Server Error error was encountered

Yup, that's the typical response when there is something wrong with the entire host rather than just some specific requested URL. Server looks for
subdomain.example.com/blahblah
encounters an error and therefore looks for
subdomain.example.com/custom500
running into the identical error.

Specific questions may need to be directed to the subforum concerned with your specific server, Apache or IIS.