> curl, for example, seems to illegitimately strip a trailing question mark (could be only for the command line, didn’t test library usage).

umm what? I don't know what they're actually sending where they think this, but if you think curl is broken you should re-think that maybe you're the one doing something wrong.

Here are some examples showing curl not stripping question marks (obviously), I am very curious what this person was actually seeing

    $ curl -s 'https://httpbingo.org/get?' | jq .url
    "https://httpbingo.org/get?"
    $ curl -s 'https://httpbingo.org/get?path' | jq .url
    "https://httpbingo.org/get?path"
    $ curl -s 'https://httpbingo.org/get?path,query=bananas' | jq .url
    "https://httpbingo.org/get?path,query=bananas"
    $ curl -s 'https://httpbingo.org/get????' | jq .url               
    "https://httpbingo.org/get????"
    $ curl -sv 'https://httpbingo.org/????' 2>&1 | grep :path
    * [HTTP/2] [1] [:path: /????]