Vectorizing text handling is likely to use generalized register permutes (and I spy some _mm512_shuffle_epi8 here), which are the bugaboo in length-agnostic SIMD. Fundamentally, the maximum index you can read from in a register depends on the register size.

So yeah, even in RISC-V vrgather has explicitly different per-element operation depending on VLMAX, which obviously depends on the HW's VLEN. So depending on the table size, you have to assume constraints on VLEN or execute different permute sequences.