"The use of civilians as human shields is not novel.8 Evidence of the practice dates back to the American Civil War9 and the Second World War.10 The practice has also been documented in the Korean Conflict and the Vietnam War. 11 United Nations (U.N.) peacekeeping forces similarly faced attacks from weapon systems placed within civilian areas or hostile forces that used civilians as human shields, for example, in Beirut in the early 1980s and Somalia in the early 1990s.12 The human shields tactic was also employed by Saddam Hussein’s Iraq in many of its conflicts.13"

https://law.stanford.edu/wp-content/uploads/2018/03/rubinste...

Countercitation: use of humans as literal shields is a fairly old tactic employed e.g. by the Mongol Empire: https://en.wikipedia.org/wiki/Military_of_the_Mongol_Empire#...

Sorry, I should say no one has used their own people as human shields before. The mongols used captives from the other side.

If there is a 1:400 ratio of IDF soldiers to civilians somewhere, is it a legitimate target?

What do you mean civilians?