Chlorinated water adversely affects PEX pipes. I don’t know that the amount of microplastics or nanoplastics has been quantified in a study yet.

https://pubs.acs.org/doi/10.1021/acs.est.9b03673

The mechanical properties and lifetime of PEX were reduced after exposure to chlorinated water. (6) Prompted by concerns about the effect of chlorine dioxide on the chemical integrity of pipe materials, the pressure, tensile strength, and oxidation induction time were evaluated at constant temperature to assess the damage to pipe samples after exposure to chlorine dioxide for one year. (7) Overall, pipe aging due to long-term disinfectant exposure could cause decreased antioxidant contents; increased crystallinity; chain rupture; hydroxyl, carbonyl, and/or vinyl group production; and visible cracks in pipe walls.

Can such an aging process lead to MP and/or NP leaching into the drinking water network? On the basis of aging mechanisms and material performance characteristics, we propose that MPs and/or NPs can be leached from aging pipes.

The Brita pitcher comment is about my own growing paranoia about plastics that get scratched or cracked. I don’t know the conditions and time line under which this particular formulation of plastic keeps it from shedding MPs.