Sending REST response in plain text or JSON?
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When creating a REST API, if I return data in JSON format for GET requests, is it a good practice to also send responses for POST, PUT, DELETE, and error messages in JSON too or plain text will suffice?

For example:

For POST request where I need to return the new ID of the newly added record (i.e. just the new auto increment value in database), should the response still be in JSON or just plain text like "1" or "2" for example.

Same goes for PUT or DELETE request where I need to say "OK" or "Accepted", as well error messages like "Bad Request", "Not Found", etc.

Plain text or JSON?
