Which one is correct, "!?" or "?!"?

Centrox's proof is lack of evidence that the interobang is used formally.

In 1968 an Interrobang key was available on some Remington typewriters, and into the 70s, some Smith-Corona typewriters incorporated it, although not a standard mark. In the 60s, it was considered quite en vogue and consequently appeared in some dictionaries and was used in various newspaper and magazine articles.

https://dgkayewriter.com/punctuation-familiar-interrobang/

InfraBlue wrote:

Centrox's proof is lack of evidence that the interobang is used formally.

Mr Standard Formal English even spelled it wrong. centrox gave no proof. He rarely does because searching for proof will make him realize just how little he knows about how the English language works.

Quote:

In 1968 an Interrobang key was available on some Remington typewriters, and into the 70s, some Smith-Corona typewriters incorporated it, although not a standard mark. In the 60s, it was considered quite en vogue and consequently appeared in some dictionaries and was used in various newspaper and magazine articles.

https://dgkayewriter.com/punctuation-familiar-interrobang/

There are no misspellings in what centrox wrote.

It's[sic] use by a few publications was a passing fad according to your article. This as well doesn't contradict what centrox had asserted.

Clearly, you have no notion of how proof works.

Quote:

There are no misspellings in what centrox wrote.

Two separate sentences, Infra. I thought being a know nothing prescriptivist was supposed to make one better at English.

InfraBlue wrote:

It's[sic] use by a few publications was a passing fad according to your article. This as well doesn't contradict what centrox had asserted.

As I said, it now exists for all to use, in SWE and in informal writing. Neither centrox nor you know how much it is used. I suspect not much, which is confirmed in the M-W description.

One thing is that the typeface character exists for all to use. Quite another thing is the interrobang's use in formal writing. Except for the few instances referred to in the article you dredged up, it hasn't been used in formal writing. It's use never caught on. It's limited, faddish use did not establish a standard for its formal use.