Where are custom ArcObjects geoprocessing tools used?

[Bud]

The Use a custom geoprocessing tool docs say:

Custom geoprocessing tools that are built with ArcObjects are not supported in ArcGIS Pro.

If custom ArcObjects tools can't be used in ArcGIS Pro, what ArcGIS product are they used in?

[AlfredBaldenweck]

I'd assume tools built for ArcMap or earlier tech. This reads like another "hey legacy stuff doesn't work anymore" notice.