> Do you know of any other uses for bytes that are not 8 bits?

For "bytes" as the term-of-art itself? Probably not. For "codes" or "words"? 5 bits are the standard in Baudot transmission (in teletype though). 6- and 7-bit words were the standards of the day for very old computers (ASCII is in itself a 7-bit code), especially on DEC-produced ones (https://rabbit.eng.miami.edu/info/decchars.html).

Back in the days of Octal notation, there were computers with a 12 bit word size that used sixbit characters (early DEC PDP-8, PDP-5, early CDC machines). 'Byte' was sometimes used for 6- and 9-bit halfword values.

I wanted to reply with a bunch of DSP examples but on further investigation the ones I checked just now seem to very deliberately use the term "data word". That said, the C char type in these cases is one "data word" as opposed to 8 bits; I feel like that ought to count as a non-8-bit byte regardless of the terminology in the docs.

NXP makes a number of audio DSPs with a native 24 bit width.

Microchip still ships chips in the PIC family with instructions of various widths including 12 and 14 bit however I believe the data memory on those chips is either 8 or 16 bit. I have no idea how to classify a machine where the instruction and data memory widths don't match.

Unlike POSIX, C merely requires that char be at least 8 bits wide. Although I assume lots of real world code would break if challenged on that particular detail.