Microplastics shed from synthetic fibre clothes probably stay in the mulch, crushed rock and sand and don't reach the roots of the vegetables and trees and don't feature in the runoff (from sub surface water travel) at the base of the slope.

Mulch on garden with microplastics is likely ok as plants extract the minerals | elements they need.

I'd have to sample and check.

A great deal of our clothing, bedding, towels, etc is natural fibre in any case.