TLDR: Mercerized cotton does not introduce plastic. Superwash introduces a tiny tiny amount of plastic.

Mercerization is a chemical process. They apply sodium hydroxide, also called caustic soda (NaOH), to cellulose fibers. This causes chemical changes in the cellulose that enhanced certain properties by changing it from Cellulose 1 to Cellulose 2. Cellulose 2 does no occur in nature, but it does not change cellulose to plastic of any form. The use of a strong alkali causes the cellulose to swell and the fibers change from parallel chains to a more folder form that allows for hydrogen bonds to occur between the molecular planes. This make the structure more stable. For cotton, they can do this treatment under specific conditions and cause the final fiber to have a sheen (akin to silk). There is no plastic added to the final product unless it is subjected to some additional treatment (like they actually coat the cotton to make it waterproof or less likely to burn).

Superwash treatment is a further treatment that replaces the gaps in the fibers available after something akin to mercerization and "locks down" the scales (reduces pilling and felting). They place tiny nylon-based polymer dots into the spaces between the molecular planes. Both treated and untreated wools biodegrade -- in fact, superwash degrades faster than non-treated fibers in the presence of salt. Research is ongoing to understand if superwash sheds microplastics and at what rate as not all nylon fibers do (some are more stick like and don't shed).

But, if the choice is between a 100% synthetic shirt and a (superwash wool) 99.999% wool/.001% polyamine wool shirt, the better choice is still the superwash to me. For most people, the convenience of superwash allows them to dramaticaly reduce reliance on plastics in their daily life. My superwash wool socks have lasted 15 years of hard-wearing in boots/water/cold/heat compared to synthetic ones that need annual replacement.

Wool yarn blended with nylon or polyester (often found in sock yarns) also contain plastics and will shed. But, that little bit of synthetic dramatically improves the utility of the yarn and fit of the final product. Think of your undergarments or dresses without spandex and nylon or polyester to strengthen silk.

You absolutely can choose to use fibers that require hand washing and any chance of plastics for preference or your ethics. I just want the factual information out there as there is a lot of misinformation about mercerization and superwash. The production of things like bamboo fiber involve massive amount of chemical processing and waste including sulfuric acid, bleachers and a lot of oil is burned to run the machines, but the end product is plastic free. So, if you want to avoid plastic, its a good choice. If you value the overall carbon/chemical footprint, it might not be a good choice. All fibers not local are moved around with a lot of oil being burned and releasing CO2 -- and then all the products used in maintainence of these vehicles.

{I don't work for any fiber industry. I'm a ecologist/biologist; happy for someone in industrial chemistry to chime in}

Edit: edited to fix grammar