Does “Circle(px,py d=d)” syntax for geospatial dat

Question

we're using Solr in our service. Recently, we're working on upgrading Solr to Solr6 & spatial4j-v0.6. But we faced that geospatial data cannot be fed, it's rejected, that has syntax like Circle(lat, lon, d=d).

It seems the syntax is dropped from Solr, but is it really dropped? Since when and why?? I couldn't find official statement that they dropped this syntax.

As for now, I find an option format=LEGACY for the fieldType, then Solr accepting data. But, are there any recommended, proper way to do with Circle()? Should we move to WKT/GeoJSON?

<fieldType
 name="location_rpt" 
 class="solr.SpatialRecursivePrefixTreeFieldType"   
 spatialContextFactory="org.locationtech.spatial4j.context.jts.JtsSpatialContextFactory"
 format="LEGACY"/>