According to the release notes, a major difference between the v2020 and v2021 versions of SAM is the default assumption that all PV modules have an anti-reflective coating. Is there a way to turn that assumption OFF in v2021 (without going back to the v2020 version) of SAM?
It is not possible to "turn off" the anti-reflective coating assumption without modifying the source code and building your own custom version of SAM. For reference, here is the relevant code.
The anti-reflective coating option is set by the last parameter with value of 'true' in the calculateIrradianceThroughCoverDeSoto() function here: