Now with the advent of HYPER speed PCIe 4.0 NMVE m.2 drives such as the Samsung 980 Pro etc, that are cable of almost double the throughpout of PCIe 3.0 NMVE m.2 drives, does it still make sense to have a physcally seperate SSDs for scratch and cache in LR Classic?
One thing to try is to run Task Manager, watch the Disk throughput graph in the Performance tab, and while you use Lightroom Classic as intensively as you can, note the maximum read/write throughput that happens while using that application. That will tell you where the point of diminishing returns is for buying faster storage.
(Hint: You'll probably find that most applications, including graphics/video and backup, never take full advantage of extremely fast storage due to real world factors such as file system overhead or bottlenecks elsewhere in the process.)
Excellent comments, @Conrad C . I add to your parenthetical comment at the end that many parts of Lightroom Classic do not benefit from increased disk speed, and the bottleneck is CPU and/or GPU speed. Which is why I asked the OP what problem he is trying to solve, it could be a problem that is not solved by faster disk speed.