It was invented for the teletype. https://en.wikipedia.org/wiki/Backslash:

“What may be the first documented reference to the character is a 1937 maintenance manual from the Teletype Corporation, which includes a photograph showing the keyboard of its Kleinschmidt keyboard perforator WPE-3 using the Wheatstone system. In a 1945 document, Teletype used the name "diagonal key" for the symbol, and gives it a (non-standard) Morse code of ._.._ . Neither document gives its usage”

> If a left handed person does it, its been invented beforehand

I disagree. A left-handed person writing a slash in mirror image doesn’t write a backslash; they write a slash using a different glyph.