This is more of a Julia question than SymPy, as the unicode subscripts are limited to what they are (on my version of Julia):
\_( \_- \_2 \_5 \_8 \_a \_e \_i \_l \_o \_r \_schwa \_v
\_) \_0 \_3 \_6 \_9 \_beta \_gamma \_j \_m \_p \_rho \_t \_x
\_+ \_1 \_4 \_7 \_= \_chi \_h \_k \_n \_phi \_s \_u
You can certainly create symbolic variables with these unicode strings:
julia> sympify("aᵢⱼ")
aᵢⱼ
(Where the part in the string is entered a\_i[tab]\_j[tab].)