The correct syntax is:
formatter=xml
For example:
https://api.tfl.gov.uk/Line/47/StopPoints?formatter=xml
https://api.tfl.gov.uk/StopPoint/Search?query=Elephant&maxResults=5&formatter=xml
As you mentioned, you can also do this with the HTTP Accept
header e.g.:
$ curl -H "Accept: text/xml""https://api.tfl.gov.uk/Line/victoria/StopPoints"