Two ways to search history commits, depend on what you remember.

By commit message

git log --grep="variables" --oneline

Only commits whose message match the word:

By code change

git log -S "variables" --oneline -- src/

• -S : find commits that added or removed the string
• -- src/ : limit to a path

Find the commit that introduce or remove the code:

add -p to see the diff too.

See you again ^_^