So here is a screen shot of the properties of a dataset note it is UNCOMPRESSED and has a SPATIAL INDEX
When I run the following code in the Python console in ArcPro 3.0 I get the expected results
desc = arcpy.Describe('centerline_reprojected')
desc.hasSpatialIndex
True
Now... I run the compress file geodatabase tool to compress the dataset, and to prove I did this here are the properties
Now if I run the same bit of code I get False
desc.hasSpatialIndex
False
Finally if I un-compressed the data it clearly states in the properties that the spatial index exists.
It would good if this method reports the presence of the index correctly. If there is some technical reason why this can't be done the Help file needs updating to explain that it won't report the existence of a spatial index.