On iMacs with a 128GB SSD blade, one can evenYes. I split the Fusion drive shortly before install, formatting the now split HDD with HFS+, the SSD obviously formatted with APFS.
Not that I care much for the HDD speed, I kept it mainly to avoid the iMac being unhappy. From what I read, you'd need to remove the SATA cable altogether, so why not leave it? I may use it for keeping additional backup copies of important work next to my external USB time machine backups.
- split the FD,
- format it with hfs+,
- format the SSD blade with APFS,
- install macOS there,
- copy the user folder to the HDD,
- move the user folder to the HDD.
You get a really flying iMac without need to open the glued screen.