I heard somewhere that T8 was the original size for 8' fluorescent lamps. If that is true, why can't I find any F96T8 magnetic ballasts?

They did. It's difficult to find without knowing the exact part numbers however:

F96T8:
1L: SM-151-S-TP
2L: SM-2E51-S-TP

These run lamps at 200mA, I believe they are for the original 8ft T8 slimline lamps but modern 230mA F96T8 lamps will run fine on them.

F96T8/HO:
1L: R-2S65-TP MARK III
2L: R-2S86-TP MARK III

Both seem to be fairly rare.

These are all Advance part numbers, I'd imagine Universal probably made them as well but the models aren't as well documented online as Advance.

Here’s one:

https://www.ebay.com/itm/335922308969

That’s kind of cool that they made magnetic rapid-start ballasts for F96T8/HO lamps. How come they didn’t make a magnetic instant-start ballast for the 1990s 59W F96T8 slimline lamp?

They did, SM-2E51-S-TP is the Advance part number for Advance's 2 lamp 120v F98T8 59w magnetic ballast.

I have a feeling Magnetek/Universal probably made one as well but their documentation of older products isn't as available online compared to Advance's.