A GM Dealer in Canada posted this video that gives a production update on the 2021 Yukon. There's going to be 3 production phases for GMC:
Phase 1 (first 9-12 weeks of production) will be the Yukon SLT and Denali only. Only 10% of those vehicles will be SLTs and the remaining 90% will be Denalis.
Phase 2 will be the SLE and AT4 trims
Phase 3 (Approx. 20 weeks from production) will be the diesel Yukons
He also points out that the Denali will have a completely different interior layout than the other Yukon trims and the Tahoe.