As noted, Apple has said nothing about any iMacs other than that the Intel models will be replaced with M-series SoCs.
But there has been plenty of ancillary data released from multiple sources, some with very high "hit rates" on their claims, that the M-series iMacs would be larger than their Intel predecessors. We have had rumors of a 23-24" M1 iMac since early 2020 and rumors that there would be an iMac larger than 27" started gaining traction in January 2021 with l0vetodream and Bloomberg (though I now believe the J456 and J457 model numbers Bloomberg referenced are for the 2-port and 4-port 24" iMac).
The only claim I can recall about there being a direct 27" replacement for the mid-2020 iMac 5K was Ming Chi Kuo, who in March 2020 said Apple was working on a new iMac Pro that would have a 27" miniLED display that he said would launch in late 2020 alongside a 24" M1 iMac and a miniLED 12.9" iPad Pro. (Assumption at the time was the iMac Pro would be an Intel-powered model using the new W-Series Xeons Intel had released.)
Oh, so they killed it because: Intel. Hmm..
I wonder how much Apple gets to the point it's breathing hard, before they kill it. (Although the Cube could be said to be something that shouldn't have been released, although it does look AWESOME!)
EDIT: And at the uni that I went to, I asked the campus rep if I should wait it out for the IIcx replacement, or just buy the IIcx. They *would not leak any plans*. I waved cash under their noses. Not a peep. Just over a month, the IIci was released. I was, I thought, in the 'in crowd', and they would at least have a 'tell' so I could figure out what to do.
I loved the IIcx, but the dirty roms and all, well... It was a disappointment. And steve not releasing the new roms in a SIMM that the IIcx was designed to handle was also disappointing. Sad... People, at the uni, were dumping IIcx like a bad habit. I had less than zero chance to get anything out of trying to sell it. *shrug* I went to Windows, and, well...???