No, not in any normal sense. Plastics are extremely stable. Unless you're grinding your retainer down to <5microns, your retainer isn't a source of microplastics.
The pieces we're talking about either need to start small or need to be exposed to some sort of wear system - sun light, waves grinding plastic bottles together that can break down the polymer chain (actually rather hard to do).
The sunlight one[0] acts on timescales of years/decades, which is a problem for ocean and dumps because all those plastic bottles, we're starting to pay the price environmentally.
But personally, no your retainer is not a source you should be concerned with.
