Flint Knife can obtain string from sheep almost infinitely, but sheep's wool will not disappear. So you can get almost infinite wool if you have enough gravel. It is a little unbalanced. I think sheep can be sheared if a player used a Flint Knife on them too many times.

added to latest 1.17.1 and 1.18 version. Hook knife takes damage and "shears" the sheep having a builtin delay as a result.