For multicolor this is not needed. All extruders have same direction anyway.
If you start misusing it for genral porpose controlling of more motors that is of course not enough. Since it is in first place a printer firmware we take special carer that this works best. Here we support 8 bit platform so 8 bit variables have some benefits as this can be checked much faster. And for deltas we need to preserve as much memory as possible.
Having said this, for 32 bit platforms this could be extended to 32 bit variable at no cost allowing 16 axis. Same could be done for 8 bit, bit it should fall back to 8 bit as soon as no extra axis are used to preserve memory and speed where needed. I think fast8_t is 32 bit on due anyway, but for switching dynamically you would need a additional type.
