IS a dedicated scratch disc needed?
Probably 15 years ago yes, today no. Moore's Law has shown that we have enough computer power.
I'm not sure it's a matter of Moore's Law.
The issue is to get fast access to the scratch disk. With mechanical hard drives, speed of access is determined by drive throughput and drive seek time. If other applications (or Windows) are using the same drive, then when Photoshop wants the scratch drive the head may have been moved to another file somewhere else on the disk, so seek times are higher. With an SSD, seek times are approximately zero, so it doesn't matter so much if something else is using the same drive. There is just the matter of throughput, and of course if other applications are using the same drive there could be contention for that throughput, but this is a smaller issue than seek times.
The drives with the fastest throughput are NVMe SSD drives. These use PCIe rather than SATA interfaces, so have a potential throughput of up to 32Gbs rather than SATA's maximum of 6Gbs. Typical NVMe SSDs have a throughput of 20-30Gbps. The best solution is probably to have an NVMe SSD C drive, which can also be the Photoshop Scratch drive. There's probably little advantage in a separate Photoshop scratch drive, so long as the C drive is an NVMe SSD.
Most NVMe drives have a so-called "M.2" socket interface, and modern motherboards usually have one or two M.2 sockets. If not, one can get PCIe adaptor cards with an M.2 socket on them, on which one can mount an NVMe SSD drive.