2011 or 2015 Djokovic ought easily to at least replicate their wins at AO, WI, or UO, sure. Could give Nadal a battle royale at Roland Garros, certainly, but I don't know that Nadal is so much worse than he was that I'd say Djokovic would "walk" it. Perhaps Djokovic would be slight favorite, but not prohibitive favorite, I don't think.
Pretty sure Djokovic of 2012 or 2013 would win AO, WI, and UO in 2021, too, TBH, unless there were a gale in New York. But even in the gale, he almost beat Murray and I think he'd be able to take this year's crop. I would have said Djokovic of 2014 would win AO, WI, and UO in 2021, and I think he would win AO - it took an immense performance from Wawrinka to stop him. But the Djokovic who lost to Nishikori at the US Open could definitely fail to win the 2021 UO even against the current field.
He's still really good. But he's not the player he was in 2011 or 2015 anymore, and only his serve has improved really. I think 2011 Djokovic walks the Grand Slam.