Everything I know about floppy disks

That's not really true – an (E)CKD record is fundamentally the same thing as a sector. The difference is that on standard hard disks (what in IBM nomenclature is called FBA DASD), the norm is every sector on the disk has the same size; CKD permits each track to have a different sector size, and even different sized sectors on the same track. Also, CKD optionally has separate key and data fields, with a physical gap between them; in floppies and standard hard disks, there is only count and data fields (count = sector header), and any key-data separation has to be marked logically (by a byte offset) not physically.

> So I'm wondering if CKD was ever a thing on floppies?

No, CKD was never used for floppies. In the standard IBM floppy format, the sector size is in the header of each sector, so in principle you could have a mix of different sized sectors on the same track, like CKD permits; in practice, I've never heard of anyone doing that, I question whether the floppy disk controller could cope with it; even if the FDC somehow could, almost all software would be confused by it. (I suppose if the FDC can handle it, someone may have adopted it as part of a floppy disk copy protection scheme.)

Something which did actually happen, albeit rarely, was formatting the first track on the disk as 128 byte sectors, and the remaining tracks with bigger sectors (such as 256 or 512 byte), to retain some limited backward compatibility with older systems that only supported 128 byte sectors; at least being able to read the first track might make clear it was a valid disk that just couldn't be read on this system, as opposed to thinking it was an invalid disk, and then formatting it in response, causing the loss of that valid data. Systems that did this sometimes also physically recorded that first track as FM, while using MFM for the remaining tracks. This was largely a 1970s concern; by the time the 1980s came around, systems which only supported 128 byte sectors had become practically irrelevant.

The classic IBM 3740 floppy filesystem supports record-oriented files. Some vendors other than IBM adopted it (e.g. Olivetti), but most non-IBM vendors ignored it, and developed their own filesystems instead (e.g. the CP/M filesystem, Microsoft's FAT). ECMA standardised an ASCII version of it as ECMA-91 in 1984 [0]. That standard didn't support keyed/indexed files, but could easily have been extended to support it–if anybody had cared to do so, as I said, relatively few vendors ever paid attention to it. The 3740 did support a key field in each database record (sector), but the key-data separation was only marked logically (by a byte offset into the data), not physically by a recording gap.

[0] https://www.ecma-international.org/wp-content/uploads/ECMA-9...

However, it seems you might actually be right in practice. I think early 3.5" definitely did this, but while googling to determine what orientation of the hole triggers the index pulse came up with nothing, except a few patents about how you can fake an index pulse by using a timer and a speed sensor instead.

So maybe because almost nothing was actually using the index pulse anymore when 3.5" drives were introduced because almost every modern system (for the time) had settled on soft-sectoring, it's entirely possible that I've always been mistaken and there's never been a standardised index pulse for 3.5" drives between different drives relative to the "start of the track".

I guess this might explain why even though the Amiga allowed you to read the INDEX signal, it actually didn't use it and instead read a whole track at once starting when it saw a particular sync word.