Sure, but then you're wasting/losing two slots to use one or the other type of RAM, and you certainly can't populate all of the RAM slots and use two different RAM types together. It would essentially be a waste of space on a motherboard. I'm almost positive that 99.999% of the consumers out there would prefer to be able to use all four RAM slots rather than have the choice of being able to use both types of RAM on the same board with only half of the # of RAM slots functioning. The only scenario I could see this possibly being useful is for those who frequently swap RAM for testing. Other than that, RAM just isn't one of those components that is frequently swapped out or dicked around with by your average computer user, so it would be a waste, especially the cost.
If anything, it would be a better idea just to make a board that supports both DDR4 & DDR5 instead of only making half of the slots work for one or the other.