You can't export FloatCurves as .csv or .json.
You can reimport as .csv or .json.
Found in 4.24.2 CL#10758860
Reproduced in 4.23.1 CL#9631420, 4.25 Main CL#11318998
1. Create a float curve.
2. Right click->Asset Actions->Export.
Result: You can export as .copy and .t3d
Expected: You can export as .csv and .json.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-88655 in the post.