If particular syntactic categories were contributing to the overall effect of surprisal more than others, including such random slopes would lead to additional variance being explained.
this was not the case: inclusion of by-POS random slopes of surprisal did not lead to a significant improvement in model fit (PSG: χ 2 (1) = 0.86, p = 0.35; RNN: χ 2 (1) = 3.20, p = 0.07).