Does It Work With an Old Takeout Export?
Yes. Google has changed the sidecar naming over the years, and both old and new are understood.
An export from 2019 looks different from one made today. The sidecar files were named photo.jpg.json then and photo.jpg.supplemental-metadata.json now, with several variations in between.
That is why an old guide can fail on a new export, and a new script can fail on an old one. The data is the same; only the names moved.
Takeout JSON Metadata Fixer handles every naming style Google has used, including the truncated ones. It runs on your own computer and is free for your first 500 photos.