Probably a feature request.
Within
/data/minecraft/tags/entity_types/
there is no tag for controlling which entities produce milk when right clicked with a bucket.
Other tags in this folder control:
- Which entities cause a creeper to drop music discs
- Which entities glow when ringing a bell
- Which entities walk on powder snow
To name a few.
In 21w13a a third, non-cow, entity has been added. It is plausible that the omission of this tag could be a missing feature.