ThinkPad e590 48GB RAM upgrade
If you are wondering whether you can fit your e590 with a 32GB SO-DIMM DDR4 module, the answer is probably yes.
The supporting evidence is my i5 e590, which is currently running a 16GB 2400MHz module (CT16G4SFD824A.M16FE) side by side with a 32GB 3200MHz module (CT32G4SFD832A.C16FB), totalling roughly 48GB.
The dmidecode Linux tool reports that both are configured to run at 2400MT/s. Apparently the newer 3200MHz modules, which happen to be cheaper and more available than their slower predecessors, run just fine at lower frequencies.
A note on installation: When I first inserted the two modules, the laptop wouldn't boot. After waiting about 30 minutes and a couple of restarts, I removed the 16GB module and placed the 32GB module in its slot. This configuration booted in under a minute. Finally I added the 16GB module, effectively swapping the first configuration, and the computer booted successfully with 48GB of RAM.