Well Synopsys has offered full 7nm DDR5 and LPDDR5 memory interface IP since october 2018, and Cadence does as well. So Joe 7nm garage SoC builder have been able to buy this off the shelf for half a year already. If AMD doesn’t support those memory types it’s by choice. Rather dubious choice IMHO given that they try to carve themselves a market niche by supplying CPUs with high core counts and APUs with theoretically high performance. They need the data flow to support that to really reap the benefits. Or rather, their customers do.Faster ddr4 support would be a good start already. It's quite a pity the Ryzen 3x00U chips don't improve on that. ddr4 so-dimms up to ddr4-3200 (with non-overvolted 1.2V) are nowadays easily available. Although they are stuff you'd never find in notebooks pre-installed, the budget so-dimms go up to ddr4-2666 too, that would easily get you another couple percents in graphic workloads. But as I mentioned, higher memory speeds require higher Ryzen SoC voltage at some point, and maybe 2400 is the limit for whatever voltage the 15W chips use.
IMHO there's no way for lpddr5 support for the Ryzen 7nm APUs (which will appear only next year, but that's still too early), we'll first see this memory in smartphones.