Actually the server side detection in [0] isn't really affected by putting tee in the middle... and neither does -x, of course.
Good point about -x being fallible to an adversarial script, even a simple set +x would be enough!
Where's the link where this has been mentioned? I missed it.
0: https://www.idontplaydarts.com/2016/04/detecting-curl-pipe-b...
This has all been mentioned in the linked comment thread