You will soon be able to see 1921 Census hints show up under the category of "new collections" on the hints page. Above is a photo of my husband's Ancestry family tree hints page. So far, only 4 "new collections" hints have appeared but there's a reason.
Remember, to stimulate the hints system, you need to click on your ancestors and do a little searching. There are several dozen UK ancestors on the tree, but I've only researched four of those folks in the past week since the 1921 Census went live on Ancestry.
So now I'm researching those UK residents and stimulating the hints system to list additional suggestions. I like that the 1921 Census is highlighted under "new collections" and hope Ancestry will continue doing that with major new databases it adds.