It's available with the R5 but it is not called Preview AF. It is a leftover from the DRLS's. Initial AF on page 5 of the menus.
With a DLSR it worked this way. You can selected a single point and place anywhere in the VF. I kept it the middle. You use that for initial AF. You have to physically move the camera to get it on your subject. Or if you placed it to one side it activated when the subject entered the area. When AF initiated it dropped out of being the initial AF point and became part of the AF array. Like the rest of the AF points it would flicker on and off tracking your subject. It would only re-appear when AF stopped and the cycle repeated.
For mirrorless Canon adapted it to how the Preview AF worked but it was still called Initial AF point. With the newer bodies like the R6II, R7, R3 and latest I presume Canon dropped that menu, moved and renamed it.
I don't know if the R6 has that menu on page 5? I'm guessing not.
