Filtering out microplastics is not particularly difficult. I just checked the first water filter brand that came to mind, and their independent test results [0] reported no detectable microplastics after filtration [1]. Having said that, the linked website above does not inspire much confidence. They have independent lab testing, which is good. However, that testing appears to only cover bacteria; and even their, only two types. Their lab testing also does not indicate any sort of methodology, and it is not clear if the lab was even attempting to test the filter, or if they were just testing the filtered water.

[0] Funded by them, and reported on their website

[1] Their sample had a known quantity of microplastics, of which they had a 75% detection rate for their control sample.