A Microsoft spokesperson tells me that "accidental human error" is to blame for missing images of "tank man" on its Bing search results.
"We are actively working to resolve this."
The incident comes on the 32nd anniversary of the Tiananmen Square Massacre.
There isn't an actual censor who approves stuff, or a list of things that are censored. Companies are expected to reason about it themselves. More broadly, a lot of things are more/less sensitive at particular points in time.
This does suggest that stuff was happening at msft, in anticipation of heightened sensitivity because of the 30 year anniversary.