Slowest transmission speed supported by TOSLINK TX and RX?
The answer is:  it depends!

It seems that most or all Toslink transmitters have bandwidth right down to DC. On the other hand, most Toslink receivers require a minimum of 100kHz for the modulation frequency for the part to function. That's too bad, because Toslink is a very practical medium for electrically isolated communication and the TX or RX hardware, as well as the fiber optic cables, are pretty inexpensive. 

I did find one RX by Toshiba that is specified for operation down to DC, the TORX1952. Here is a link to the part at Mouser (The US supplier) and the datasheet:

Toshiba TORX1952(6M,F) at Mouser.com
DATASHEET

It's not exactly cheap at $10 each for QTY 1 but it should work for low speeds.

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