10-bit has sort of been used. The General Instrument CP1600 family of microprocessors used 16-bit words but all of the instruction opcodes only used 10 bits, with the remaining 6 bits reserved for future use.

GI made 10-bit ROMs so that you wouldn't waste 37.5% of your ROM space storing those 6 reserved bits for every opcode. Storing your instructions in 10-bit ROM instead of 16-bit ROM meant that if you needed to store 16-bit data in your ROM you would have to store it in two parts. They had a special instruction that would handle that.

The Mattel Intellivision used a CP1610 and used the 10-bit ROM.

The term Intellivision programmers used for a 10-bit quantity was "decle". Half a decle was a "nickel".

That's a very interesting bit of lore, I knew those were peculiar CPUs but I never know about these details, thank you!

I grew up down the road from a GI factory, so assumed everyone knew these things ;)