It seems motherboard manufacturers will ourt both DDR4 and DDR5 based motherboards. The upcoming ASUS Z690 motherboards featuring LGA1700 socket for Intel Alder Lake CPUs have been listed by the Eurasian Economic Commission regulatory office.

Z690 is capable of supporting both DDR5 and DDR4 memory, as they act as a bridge between the two memory standards in use at the time. Important to know is that component prices are rising again, so it might be a safe bet to stick to your DDR4 memory at this time. 

Notable is that many of these SKUs come with the addition "D4", suggesting DDR4 memory slots. For example, the Prime Z690-A has two variations, one with DDR5 slots and one with DDR4 slots. There is no D4 memory option available for the ROG Maximus XIV series, which means that they will only support DDR5 memory. We can't tell from the names if any of these models have both DDR5 and DDR4 slots, but we can expect MSI and ASRock to come up with similar arrangements.

ASUS Z690 motherboards listed give you a choice - there are DDR5 and DDR4 variants