I don’t think that there ever was a rumour about that, but there were lots of people who wNted it to happen, myself included. Sort of like a new 400F5.6 or a 500F5.6, just a lot of people wishing......
I think I remember some Canon patents on that.
But it was never clear if this would become a product, neither if it would be priced consumer level or if it would become another L lens.
And with all the Canon focus on R system and RF lenses together with the rumored EOL of the 7D series, I wouldn't bet any money that we'll see something like that within the next three to five years.
I don't remember any rumors either, just a lot of hoping. My suspicion is that Canon believes this market is served by the 100-400 + 1.4x + f/8 focusing bodies.
Those rumors may be like the rumors of the 100-400 MkII...remaining rainbow-farting unicorn for many years, until they weren’t. I think a CR2 rumor of the 100-400 II was posted back when CR was sent out via snail mail.