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The f-curve for the Y location is still there, but it isn't working. I must have clicked or pressed something without realizing, but I can't find what. It shows up in white with all its keyframes intact, and the object isn't moving when I scrub.
You must have accidentally pressed the 'speaker' icon next to the f-curve in the Graph Editor, which is used to temporarily disable (mute) f-curves:
Click it to enable the f-curve again:
Required, but never shown