The custom firmware (really just a magic string) was required for "Apple HD SC Setup" or the later "Drive Setup" to recognize, partition, and format the drive.

In order to upgrade the hard drive in a classic 68k or PPC Mac you had to either buy an Apple drive, or buy a third party disk utility capable of setting up a non-Apple drive.

Many Apple-specific retailers would bundle in the software needed to use a new hard drive with the purchase of one but you'd better believe they baked the cost in to the price of the drive.

If all you had was a Mac, your System disks, and a new non-Apple hard drive you 100% could not use it.

I used FWB Hard Disk Toolkit, but it was >$100.

Both Apple HD SC Setup and Drive Setup have been patched by the retro community to work with any drive, but in the 80s and 90s this was not the case.

They were upgradable but the Apple hard disk utility would only format Apple drives. There were several third party utilities and they were usually bundled when buying e.g. Quantum drives, but if you took a drive from elsewhere you'd have to have a copy of the utility.

If there was custom firmware, I think it was just the model ID or some other license key system.

My experience was the same as you. SCSI drives were swappable. I also remember many claims at the time that they required special firmware.