I always found the information deficit on Class A airspace intriguing. When digging into the source for the false strict limitation being taught, I found that the source may be a sloppy reading of the ICAO definition.
The FAA does excellent work and so they do on this matter:
https://www.faa.gov/regulations_poli..._phak_ch15.pdf
cite: "
Unless otherwise authorized, all operation in Class A airspace is conducted under instrument flight rules (IFR)." Bold marked by me.
AVWeb did a nice article on even VFR required in Class A situations in 2000 and I suggest to read it.
https://www.avweb.com/news/system/182429-1.html