There's a possibility that GM is using a Silverado/Sierra as a mule test truck for the 2021 Tahoe/Suburban/Yukon. I would think that for 2021 a refresh would not be probable due to the high expense of switching over assembly lines, managing overseas suppliers, new tooling etc. If anything I would imagine only cosmetic changes, info screen, maybe head lights or something similar to the 2015-2016 refresh.