Edit: I've googled it and I can't find anything relevant. I've been working in software for 20+ years and read a myriad things and it's the first time I hear about it...
"Shift-left" was a general term that occurred in the systems engineering / devops space – I'm not surprised to see it used in a security context now. More or less, about a decade ago most systems engineers were recruited into the industry without any application software engineering skills and that became a drag on organizations trying to scale. It was about moving testing, devops, security, etc into the software engineering role and attempting to consolidate systems engineering into SWE roles. It was a part of the larger "devops movement".
Edit: I've googled it and I can't find anything relevant. I've been working in software for 20+ years and read a myriad things and it's the first time I hear about it...