[oasisbob]

"No true Scotsman allows GETs with side effects" is not a strong argument

It's not just HTTP where this is a problem. There are enough http-ish protocols where protocol smuggling confusion is a risk. It's possible to send chimeric HTTP requests at devices which then interpret them as a protocol other than http.

[bawolff]

Yes, which is why web browsers way back even in the netscape navigator era had a blacklist of ports that are disallowed.