Can we use the 'i' register ?

I think we can.

I use it when I’m out of registers in Lilly’s Saga. I’m using im1.

Note the great feature of flags being set when doing

ld a,i 

The sign flag and zero flag is set automatically. So, for example, a 3-way conditional can be directly performed:

jp m, xxxx
jp z, yyyy
…

as other things ofc. But always take into account that this ld-command is a bit more costly than normal.

I used it as double buffer flag on Freedom Fighter's scrolling routine

But a cool MSX OS got an IM2 interrupt server in page 3, so one can flip the slot in page 0 freely. And then the i register cannot be used.

Can you show a code that needs the i register.

And SDCC development time would better be spent on its base weakness, like seen in

p->x += p->dx;

which needs multiple registers for the addresses, when it could go in one address register (I havent checked version 4 yet).

You talk about SymbOS?

I guess they don't do this so that we can use it ourselves in our asm routines.