When working with branches, often you are focusing on a specific part of the codebase. Sometimes, a commit in a string of commits can cause failures elsewhere. Check out my script on bisecting the current branch and catch the commit that introduced the error.
The text you are looking for isn't in the current revision anymore, and so you'll have to dig through the Git history somehow, looking for when it was removed/introduced. Read about how I approached this from a CLI perspective, using a script to simplify these searches for me.
If you are going to edit commits in a collapsing fashion (i.e., amend/squash) in a pull request, make it your last action before merging to avoid confusing code reviewers.
In a large long-lived git repository there is a lot of history behind the current state of things. git blame lets you scratch the surface, while git log lets you dig deeper. The use of git evolution provides a succient and yet comprhensive approach to understanding the evolution of a range of lines within a git repository.