Typically, platforms and scaffolding, must comply with OSHA regulations. I don't recall there being any specific welding code or minimum qualifications that must be used, except when contract docs specify otherwise. Ultimately the company owners and supervisors of the persons who do the welding are responsible.
You bring up a good point though. I personally know of 2 men being killed when a staging bracket welded to the side of a storage tank let go, due to poor welding. It broke suddenly, and the men fell 65 feet. That was 30 years ago. No one ever considered tie-off, railings, or much of anything else back then. Thankfully, times have changed. It isn't perfect, but definitely better.
Chet Guilford
When structural contract specifications do reference ladders and platforms, which is rare, they call out OSHA 29CFR, Part 1910 (walking/working surfaces). I dug up one specific document calling out 1910.27 (fixed ladders) it lists AWS D1.0-66 as the welding specification.
I have not seen this question addressed in ASME or API applications unless the ladder clip is attached directly to the vessel or tank shell.