In short, they won't put a higher end sensor in a lower-end body, so it is simplified in most ways, including the sensor.
The main difference between them is probably the readout speed, but it needs the processors to keep up, with it and that adds heat, size and cost. So it works in a camera that cost 3x more and also 3x bigger, but not in the EOS R6.
So the sensor is slower, but not as much as the older generation (and less expensive, needs less processing power to use it) but some advantages still remain.
Like the ISO sensitivity for stills (you also get a clearer view in the EVF)and lower framerate 4K video, and low-light AF and it will still have good video capabilities, it actually becomes is less crippled for video with less megapixels (where I think the IBIS is even more useful, since for capturing action the IBIS is not so beneficial)
The R5 looks like a beefed up EOS R and the R6 like a beefed up EOS RP, but they all remain in the product range.