> It is also certified for Particulate Class 1, which is a surrogate for microplastics.

Based on a web search, it looks like Particulate Class 1 means particles in the 0.5 to 1 micron range. Several carbon block filters are rated at 0.5 µm, so I guess they're meant to handle those microplastics, but it leaves me wondering:

Do any smaller microplastics exist? Are they likely to be present in municipal water supplies?

All the filter cartridges that I've seen, and almost all the housings and tubes that hold them and the water, are made at least partly of plastic. Given that water typically sits in these filtration systems for hours or days at a time when the tap is closed, could it be that microplastics are leaching into the water from them?