How do I search git history for a disappeared line?

This is a job for the pickaxe!

From the git-log manpage:

-S<string>

Look for differences that introduce or remove an instance of <string>. Note that this is different than the string simply appearing in diff output; see the pickaxe entry in gitdiffcore(7) for more details.

You can of course use further options to narrow it down, for example:

git log -Sfoobar --since=2008.1.1 --until=2009.1.1 -- path_containing_change

There is also:

-G<regex>

Look for differences whose patch text contains added/removed lines that match .
To illustrate the difference between -S –pickaxe-regex and -G, consider a commit with the following diff in the same file:

+ return frotz(nitfol, two->ptr, 1, 0);

...

- hit = frotz(nitfol, mf2.ptr, 1, 0);

While git log -G"frotz\(nitfol" will show this commit, git log -S"frotz\(nitfol" --pickaxe-regex will not (because the number of occurrences of that string did not change).