Workflow question

It depends on your checkbook. Ideally, you have fast SSD storage, either NVMe or Thunderbolt 3/4, and separate drives/RAID for everything with dedicated external (and possibly cloud) backups. That can get costly in a hurry, though.

I think such proposals are justified, since sooner or later many come to a similar configuration. The use of different disks for different stages of work is advisable both from the point of view of protecting data from loss and from the point of view of selecting the optimal components depending on the tasks.

In my understanding:

• The system drive is fast SSD or NVMe and reliable at the same time. The volume of the disk should be enough for the operation of the operating system and the necessary programs. It is desirable that there are no user files on it (that is, a sudden failure of the system disk should not lead to the loss of your data). Operating systems allow the use of symbolic links for easy access to user data located on different drives - it's easy enough to transfer folders with presets, fonts, etc. to another disk.
• The cache drive is also a fast SSD or NVMe, but not necessarily a high price segment. The volume is usually less than the volume of the system disk. I usually take the cheapest disks for these purposes or use disks that have partially exhausted their resource on other computers. In other words, the loss of a short-term cache is not critical, but the speed of access to it is important.
• The disk for current projects should provide comfortable work with your current projects. It should be both reliable and fast enough that you don't have to wait endlessly for files to open. Its volume varies depending on your workflow - it is enough for someone to store data for 1 month, someone stores working files for a quarter, less often for half a year or a year. 
• The storage Disk is a disk for long-term storage of completed projects and various supporting files. That is, the data that you do not need daily, but that you need to save. The speed of data access is not so important here, but the reliability of the drive and its size are in the first place.

Just as @c.pfaffenbichler rightly noted, regardless of the configuration of the computer, it is desirable to provide a backup of all critical data. Ideally, your data should be backed up to another device (so that a total computer crash doesn't leave you without your files). NAS or DAS type devices are ideal here. The backup policy is individual - for me, for example, the system disk and storage are backed up to NAS once a month, the disk with projects is backed up every day (I use cloud storage for this). Once a quarter, I completely clear the disk with projects, transferring part of the data to the storage, and writing the rest immediately to NAS backups.

I suspect the recommendation you found boils down to: 
• one should ideally only keep OS and applications (and related resources like fonts, ICC profiles, presets etc.) on the System Drive 
• one should, if possible, use a dedicated Scratch Disk 
https://helpx.adobe.com/photoshop/using/scratch-disks-preferences.html

So the »project disk« would simply be a disk different from the OS and the Scratch Disk. 

Aside from that backing up one’s data regularly is certainly recommendable. 
Long term storage may also benefit from having a back-up.